f-strings everywhere

docs/api.rst

@@ -58,12 +58,12 @@ Incoming Request Data
       the following:
 
       ============= ======================================================
-      `path`        ``u'/π/page.html'``
-      `full_path`   ``u'/π/page.html?x=y'``
-      `script_root` ``u'/myapplication'``
-      `base_url`    ``u'http://www.example.com/myapplication/π/page.html'``
-      `url`         ``u'http://www.example.com/myapplication/π/page.html?x=y'``
-      `url_root`    ``u'http://www.example.com/myapplication/'``
+      `path`        ``'/π/page.html'``
+      `full_path`   ``'/π/page.html?x=y'``
+      `script_root` ``'/myapplication'``
+      `base_url`    ``'http://www.example.com/myapplication/π/page.html'``
+      `url`         ``'http://www.example.com/myapplication/π/page.html?x=y'``
+      `url_root`    ``'http://www.example.com/myapplication/'``
       ============= ======================================================
 
 

docs/config.rst

@@ -596,8 +596,8 @@ your configuration classes::
         DB_SERVER = '192.168.1.56'
 
         @property
-        def DATABASE_URI(self):         # Note: all caps
-            return 'mysql://user@{}/foo'.format(self.DB_SERVER)
+        def DATABASE_URI(self):  # Note: all caps
+            return f"mysql://user@{self.DB_SERVER}/foo"
 
     class ProductionConfig(Config):
         """Uses production database server."""

docs/patterns/flashing.rst

@@ -103,7 +103,7 @@ error messages could be displayed with a red background.
 To flash a message with a different category, just use the second argument
 to the :func:`~flask.flash` function::
 
-    flash(u'Invalid password provided', 'error')
+    flash('Invalid password provided', 'error')
 
 Inside the template you then have to tell the
 :func:`~flask.get_flashed_messages` function to also return the

docs/patterns/lazyloading.rst

@@ -93,7 +93,7 @@ write this by having a function that calls into
 name and a dot, and by wrapping `view_func` in a `LazyView` as needed.  ::
 
     def url(import_name, url_rules=[], **options):
-        view = LazyView('yourapplication.' + import_name)
+        view = LazyView(f"yourapplication.{import_name}")
         for url_rule in url_rules:
             app.add_url_rule(url_rule, view_func=view, **options)
 

docs/patterns/requestchecksum.rst

@@ -52,4 +52,4 @@ Example usage::
         files = request.files
         # At this point the hash is fully constructed.
         checksum = hash.hexdigest()
-        return 'Hash was: %s' % checksum
+        return f"Hash was: {checksum}"

docs/patterns/sqlalchemy.rst

@@ -104,9 +104,9 @@ You can insert entries into the database like this:
 Querying is simple as well:
 
 >>> User.query.all()
-[<User u'admin'>]
+[<User 'admin'>]
 >>> User.query.filter(User.name == 'admin').first()
-<User u'admin'>
+<User 'admin'>
 
 .. _SQLAlchemy: https://www.sqlalchemy.org/
 .. _declarative:
@@ -200,19 +200,19 @@ SQLAlchemy will automatically commit for us.
 To query your database, you use the engine directly or use a connection:
 
 >>> users.select(users.c.id == 1).execute().first()
-(1, u'admin', u'admin@localhost')
+(1, 'admin', 'admin@localhost')
 
 These results are also dict-like tuples:
 
 >>> r = users.select(users.c.id == 1).execute().first()
 >>> r['name']
-u'admin'
+'admin'
 
 You can also pass strings of SQL statements to the
 :meth:`~sqlalchemy.engine.base.Connection.execute` method:
 
 >>> engine.execute('select * from users where id = :1', [1]).first()
-(1, u'admin', u'admin@localhost')
+(1, 'admin', 'admin@localhost')
 
 For more information about SQLAlchemy, head over to the
 `website <https://www.sqlalchemy.org/>`_.

docs/patterns/streaming.rst

@@ -21,7 +21,7 @@ data and to then invoke that function and pass it to a response object::
     def generate_large_csv():
         def generate():
             for row in iter_all_rows():
-                yield ','.join(row) + '\n'
+                yield f"{','.join(row)}\n"
         return Response(generate(), mimetype='text/csv')
 
 Each ``yield`` expression is directly sent to the browser.  Note though

docs/patterns/viewdecorators.rst

@@ -142,8 +142,7 @@ Here is the code for that decorator::
             def decorated_function(*args, **kwargs):
                 template_name = template
                 if template_name is None:
-                    template_name = request.endpoint \
-                        .replace('.', '/') + '.html'
+                    template_name = f"'{request.endpoint.replace('.', '/')}.html'"
                 ctx = f(*args, **kwargs)
                 if ctx is None:
                     ctx = {}

docs/quickstart.rst

@@ -447,11 +447,11 @@ Here is a basic introduction to how the :class:`~markupsafe.Markup` class works:
 
     >>> from markupsafe import Markup
     >>> Markup('<strong>Hello %s!</strong>') % '<blink>hacker</blink>'
-    Markup(u'<strong>Hello &lt;blink&gt;hacker&lt;/blink&gt;!</strong>')
+    Markup('<strong>Hello &lt;blink&gt;hacker&lt;/blink&gt;!</strong>')
     >>> Markup.escape('<blink>hacker</blink>')
-    Markup(u'&lt;blink&gt;hacker&lt;/blink&gt;')
+    Markup('&lt;blink&gt;hacker&lt;/blink&gt;')
     >>> Markup('<em>Marked up</em> &raquo; HTML').striptags()
-    u'Marked up \xbb HTML'
+    'Marked up \xbb HTML'
 
 .. versionchanged:: 0.5
 
@@ -609,8 +609,8 @@ Werkzeug provides for you::
     @app.route('/upload', methods=['GET', 'POST'])
     def upload_file():
         if request.method == 'POST':
-            f = request.files['the_file']
-            f.save('/var/www/uploads/' + secure_filename(f.filename))
+            file = request.files['the_file']
+            file.save(f"/var/www/uploads/{secure_filename(f.filename)}")
         ...
 
 For some better examples, checkout the :ref:`uploading-files` pattern.

docs/templating.rst

@@ -222,8 +222,8 @@ functions)::
 
     @app.context_processor
     def utility_processor():
-        def format_price(amount, currency=u'€'):
-            return u'{0:.2f}{1}'.format(amount, currency)
+        def format_price(amount, currency="€"):
+            return f"{amount:.2f}{currency}"
         return dict(format_price=format_price)
 
 The context processor above makes the `format_price` function available to all

docs/testing.rst

@@ -165,16 +165,19 @@ invalid credentials.  Add this new test function::
     def test_login_logout(client):
         """Make sure login and logout works."""
 
-        rv = login(client, flaskr.app.config['USERNAME'], flaskr.app.config['PASSWORD'])
+        username = flaskr.app.config["USERNAME"]
+        password = flaskr.app.config["PASSWORD"]
+
+        rv = login(client, username, password)
         assert b'You were logged in' in rv.data
 
         rv = logout(client)
         assert b'You were logged out' in rv.data
 
-        rv = login(client, flaskr.app.config['USERNAME'] + 'x', flaskr.app.config['PASSWORD'])
+        rv = login(client, f"{username}x", password)
         assert b'Invalid username' in rv.data
 
-        rv = login(client, flaskr.app.config['USERNAME'], flaskr.app.config['PASSWORD'] + 'x')
+        rv = login(client, username, f'{password}x')
         assert b'Invalid password' in rv.data
 
 Test Adding Messages

docs/tutorial/blog.rst

@@ -207,7 +207,7 @@ it from each view.
         ).fetchone()
 
         if post is None:
-            abort(404, "Post id {0} doesn't exist.".format(id))
+            abort(404, f"Post id {id} doesn't exist.")
 
         if check_author and post['author_id'] != g.user['id']:
             abort(403)

docs/tutorial/views.rst

@@ -94,7 +94,7 @@ write templates to generate the HTML form.
             elif db.execute(
                 'SELECT id FROM user WHERE username = ?', (username,)
             ).fetchone() is not None:
-                error = 'User {} is already registered.'.format(username)
+                error = f"User {username} is already registered."
 
             if error is None:
                 db.execute(

src/flask/app.py

@@ -585,7 +585,7 @@ class Flask(_PackageBoundObject):
                 bool(static_host) == host_matching
             ), "Invalid static_host/host_matching combination"
             self.add_url_rule(
-                self.static_url_path + "/<path:filename>",
+                f"{self.static_url_path}/<path:filename>",
                 endpoint="static",
                 host=static_host,
                 view_func=self.send_static_file,
@@ -711,7 +711,7 @@ class Flask(_PackageBoundObject):
         prefix, package_path = find_package(self.import_name)
         if prefix is None:
             return os.path.join(package_path, "instance")
-        return os.path.join(prefix, "var", self.name + "-instance")
+        return os.path.join(prefix, "var", f"{self.name}-instance")
 
     def open_instance_resource(self, resource, mode="rb"):
         """Opens a resource from the application's instance folder
@@ -1084,10 +1084,11 @@ class Flask(_PackageBoundObject):
 
         if blueprint.name in self.blueprints:
             assert self.blueprints[blueprint.name] is blueprint, (
-                "A name collision occurred between blueprints %r and %r. Both"
-                ' share the same name "%s". Blueprints that are created on the'
-                " fly need unique names."
-                % (blueprint, self.blueprints[blueprint.name], blueprint.name)
+                "A name collision occurred between blueprints"
+                f" {blueprint!r} and {self.blueprints[blueprint.name]!r}."
+                f" Both share the same name {blueprint.name!r}."
+                f" Blueprints that are created on the fly need unique"
+                f" names."
             )
         else:
             self.blueprints[blueprint.name] = blueprint
@@ -1209,8 +1210,8 @@ class Flask(_PackageBoundObject):
             old_func = self.view_functions.get(endpoint)
             if old_func is not None and old_func != view_func:
                 raise AssertionError(
-                    "View function mapping is overwriting an "
-                    "existing endpoint function: %s" % endpoint
+                    "View function mapping is overwriting an existing"
+                    f" endpoint function: {endpoint}"
                 )
             self.view_functions[endpoint] = view_func
 
@@ -1341,17 +1342,18 @@ class Flask(_PackageBoundObject):
         """
         if isinstance(code_or_exception, HTTPException):  # old broken behavior
             raise ValueError(
-                "Tried to register a handler for an exception instance {!r}."
-                " Handlers can only be registered for exception classes or"
-                " HTTP error codes.".format(code_or_exception)
+                "Tried to register a handler for an exception instance"
+                f" {code_or_exception!r}. Handlers can only be"
+                " registered for exception classes or HTTP error codes."
             )
 
         try:
             exc_class, code = self._get_exc_class_and_code(code_or_exception)
         except KeyError:
             raise KeyError(
-                "'{}' is not a recognized HTTP error code. Use a subclass of"
-                " HTTPException with that code instead.".format(code_or_exception)
+                f"'{code_or_exception}' is not a recognized HTTP error"
+                " code. Use a subclass of HTTPException with that code"
+                " instead."
             )
 
         handlers = self.error_handler_spec.setdefault(key, {}).setdefault(code, {})
@@ -1730,7 +1732,7 @@ class Flask(_PackageBoundObject):
                 # message, add it in manually.
                 # TODO: clean up once Werkzeug >= 0.15.5 is required
                 if e.args[0] not in e.get_description():
-                    e.description = "KeyError: '{}'".format(*e.args)
+                    e.description = f"KeyError: {e.args[0]!r}"
             elif not hasattr(BadRequestKeyError, "show_exception"):
                 e.args = ()
 
@@ -2006,9 +2008,9 @@ class Flask(_PackageBoundObject):
         # the body must not be None
         if rv is None:
             raise TypeError(
-                'The view function for "{}" did not return a valid response. The'
-                " function either returned None or ended without a return"
-                " statement.".format(request.endpoint)
+                f"The view function for {request.endpoint!r} did not"
+                " return a valid response. The function either returned"
+                " None or ended without a return statement."
             )
 
         # make sure the body is an instance of the response class
@@ -2028,17 +2030,17 @@ class Flask(_PackageBoundObject):
                     rv = self.response_class.force_type(rv, request.environ)
                 except TypeError as e:
                     raise TypeError(
-                        "{e}\nThe view function did not return a valid"
-                        " response. The return type must be a string, dict, tuple,"
-                        " Response instance, or WSGI callable, but it was a"
-                        " {rv.__class__.__name__}.".format(e=e, rv=rv)
+                        f"{e}\nThe view function did not return a valid"
+                        " response. The return type must be a string,"
+                        " dict, tuple, Response instance, or WSGI"
+                        f" callable, but it was a {type(rv).__name__}."
                     ).with_traceback(sys.exc_info()[2])
             else:
                 raise TypeError(
                     "The view function did not return a valid"
-                    " response. The return type must be a string, dict, tuple,"
-                    " Response instance, or WSGI callable, but it was a"
-                    " {rv.__class__.__name__}.".format(rv=rv)
+                    " response. The return type must be a string,"
+                    " dict, tuple, Response instance, or WSGI"
+                    f" callable, but it was a {type(rv).__name__}."
                 )
 
         # prefer the status if it was provided
@@ -2375,4 +2377,4 @@ class Flask(_PackageBoundObject):
         return self.wsgi_app(environ, start_response)
 
     def __repr__(self):
-        return f"<{self.__class__.__name__} {self.name!r}>"
+        return f"<{type(self).__name__} {self.name!r}>"

src/flask/blueprints.py

@@ -246,7 +246,7 @@ class Blueprint(_PackageBoundObject):
 
         if self.has_static_folder:
             state.add_url_rule(
-                self.static_url_path + "/<path:filename>",
+                f"{self.static_url_path}/<path:filename>",
                 view_func=self.send_static_file,
                 endpoint="static",
             )

src/flask/cli.py

@@ -62,13 +62,13 @@ def find_best_app(script_info, module):
         return matches[0]
     elif len(matches) > 1:
         raise NoAppException(
-            'Detected multiple Flask applications in module "{module}". Use '
-            '"FLASK_APP={module}:name" to specify the correct '
-            "one.".format(module=module.__name__)
+            "Detected multiple Flask applications in module"
+            f" {module.__name__!r}. Use 'FLASK_APP={module.__name__}:name'"
+            f" to specify the correct one."
         )
 
     # Search for app factory functions.
-    for attr_name in ("create_app", "make_app"):
+    for attr_name in {"create_app", "make_app"}:
         app_factory = getattr(module, attr_name, None)
 
         if inspect.isfunction(app_factory):
@@ -81,15 +81,16 @@ def find_best_app(script_info, module):
                 if not _called_with_wrong_args(app_factory):
                     raise
                 raise NoAppException(
-                    'Detected factory "{factory}" in module "{module}", but '
-                    "could not call it without arguments. Use "
-                    "\"FLASK_APP='{module}:{factory}(args)'\" to specify "
-                    "arguments.".format(factory=attr_name, module=module.__name__)
+                    f"Detected factory {attr_name!r} in module {module.__name__!r},"
+                    " but could not call it without arguments. Use"
+                    f" \"FLASK_APP='{module.__name__}:{attr_name}(args)'\""
+                    " to specify arguments."
                 )
 
     raise NoAppException(
-        'Failed to find Flask application or factory in module "{module}". '
-        'Use "FLASK_APP={module}:name to specify one.'.format(module=module.__name__)
+        "Failed to find Flask application or factory in module"
+        f" {module.__name__!r}. Use 'FLASK_APP={module.__name__}:name'"
+        " to specify one."
     )
 
 
@@ -150,8 +151,7 @@ def find_app_by_string(script_info, module, app_name):
 
     if not match:
         raise NoAppException(
-            '"{name}" is not a valid variable name or function '
-            "expression.".format(name=app_name)
+            f"{app_name!r} is not a valid variable name or function expression."
         )
 
     name, args = match.groups()
@@ -165,11 +165,8 @@ def find_app_by_string(script_info, module, app_name):
         if args:
             try:
                 args = ast.literal_eval(f"({args},)")
-            except (ValueError, SyntaxError) as e:
-                raise NoAppException(
-                    "Could not parse the arguments in "
-                    '"{app_name}".'.format(e=e, app_name=app_name)
-                )
+            except (ValueError, SyntaxError):
+                raise NoAppException(f"Could not parse the arguments in {app_name!r}.")
         else:
             args = ()
 
@@ -180,10 +177,9 @@ def find_app_by_string(script_info, module, app_name):
                 raise
 
             raise NoAppException(
-                '{e}\nThe factory "{app_name}" in module "{module}" could not '
-                "be called with the specified arguments.".format(
-                    e=e, app_name=app_name, module=module.__name__
-                )
+                f"{e}\nThe factory {app_name!r} in module"
+                f" {module.__name__!r} could not be called with the"
+                " specified arguments."
             )
     else:
         app = attr
@@ -192,8 +188,8 @@ def find_app_by_string(script_info, module, app_name):
         return app
 
     raise NoAppException(
-        "A valid Flask application was not obtained from "
-        '"{module}:{app_name}".'.format(module=module.__name__, app_name=app_name)
+        "A valid Flask application was not obtained from"
+        f" '{module.__name__}:{app_name}'."
     )
 
 
@@ -236,11 +232,11 @@ def locate_app(script_info, module_name, app_name, raise_if_not_found=True):
         # Determine this by checking whether the trace has a depth > 1.
         if sys.exc_info()[2].tb_next:
             raise NoAppException(
-                'While importing "{name}", an ImportError was raised:'
-                "\n\n{tb}".format(name=module_name, tb=traceback.format_exc())
+                f"While importing {module_name!r}, an ImportError was"
+                f" raised:\n\n{traceback.format_exc()}"
             )
         elif raise_if_not_found:
-            raise NoAppException(f'Could not import "{module_name}".')
+            raise NoAppException(f"Could not import {module_name!r}.")
         else:
             return
 
@@ -259,14 +255,10 @@ def get_version(ctx, param, value):
     import werkzeug
     from . import __version__
 
-    message = "Python %(python)s\nFlask %(flask)s\nWerkzeug %(werkzeug)s"
     click.echo(
-        message
-        % {
-            "python": platform.python_version(),
-            "flask": __version__,
-            "werkzeug": werkzeug.__version__,
-        },
+        f"Python {platform.python_version()}\n"
+        f"Flask {__version__}\n"
+        f"Werkzeug {werkzeug.__version__}",
         color=ctx.color,
     )
     ctx.exit()
@@ -659,7 +651,7 @@ def show_server_banner(env, debug, app_import_path, eager_loading):
         return
 
     if app_import_path is not None:
-        message = f' * Serving Flask app "{app_import_path}"'
+        message = f" * Serving Flask app {app_import_path!r}"
 
         if not eager_loading:
             message += " (lazy loading)"
@@ -670,14 +662,14 @@ def show_server_banner(env, debug, app_import_path, eager_loading):
 
     if env == "production":
         click.secho(
-            "   WARNING: This is a development server. "
-            "Do not use it in a production deployment.",
+            "   WARNING: This is a development server. Do not use it in"
+            " a production deployment.",
             fg="red",
         )
         click.secho("   Use a production WSGI server instead.", dim=True)
 
     if debug is not None:
-        click.echo(" * Debug mode: {}".format("on" if debug else "off"))
+        click.echo(f" * Debug mode: {'on' if debug else 'off'}")
 
 
 class CertParamType(click.ParamType):
@@ -809,7 +801,7 @@ class SeparatedPathType(click.Path):
     type=SeparatedPathType(),
     help=(
         "Extra files that trigger a reload on change. Multiple paths"
-        " are separated by '{}'.".format(os.path.pathsep)
+        f" are separated by {os.path.pathsep!r}."
     ),
 )
 @pass_script_info
@@ -863,8 +855,10 @@ def shell_command():
     from .globals import _app_ctx_stack
 
     app = _app_ctx_stack.top.app
-    banner = "Python {} on {}\nApp: {} [{}]\nInstance: {}".format(
-        sys.version, sys.platform, app.import_name, app.env, app.instance_path,
+    banner = (
+        f"Python {sys.version} on {sys.platform}\n"
+        f"App: {app.import_name} [{app.env}]\n"
+        f"Instance: {app.instance_path}"
     )
     ctx = {}
 

src/flask/config.py

@@ -98,10 +98,10 @@ class Config(dict):
             if silent:
                 return False
             raise RuntimeError(
-                "The environment variable %r is not set "
-                "and as such configuration could not be "
-                "loaded.  Set this variable and make it "
-                "point to a configuration file" % variable_name
+                f"The environment variable {variable_name!r} is not set"
+                " and as such configuration could not be loaded. Set"
+                " this variable and make it point to a configuration"
+                " file"
             )
         return self.from_pyfile(rv, silent=silent)
 
@@ -128,7 +128,7 @@ class Config(dict):
         except OSError as e:
             if silent and e.errno in (errno.ENOENT, errno.EISDIR, errno.ENOTDIR):
                 return False
-            e.strerror = "Unable to load configuration file (%s)" % e.strerror
+            e.strerror = f"Unable to load configuration file ({e.strerror})"
             raise
         self.from_object(d)
         return True
@@ -200,7 +200,7 @@ class Config(dict):
             if silent and e.errno in (errno.ENOENT, errno.EISDIR):
                 return False
 
-            e.strerror = "Unable to load configuration file (%s)" % e.strerror
+            e.strerror = f"Unable to load configuration file ({e.strerror})"
             raise
 
         return self.from_mapping(obj)
@@ -219,7 +219,7 @@ class Config(dict):
                 mappings.append(mapping[0])
         elif len(mapping) > 1:
             raise TypeError(
-                "expected at most 1 positional argument, got %d" % len(mapping)
+                f"expected at most 1 positional argument, got {len(mapping)}"
             )
         mappings.append(kwargs.items())
         for mapping in mappings:
@@ -270,4 +270,4 @@ class Config(dict):
         return rv
 
     def __repr__(self):
-        return "<{} {}>".format(self.__class__.__name__, dict.__repr__(self))
+        return f"<{type(self).__name__} {dict.__repr__(self)}>"

src/flask/ctx.py

@@ -88,7 +88,7 @@ class _AppCtxGlobals:
     def __repr__(self):
         top = _app_ctx_stack.top
         if top is not None:
-            return "<flask.g of %r>" % top.app.name
+            return f"<flask.g of {top.app.name!r}>"
         return object.__repr__(self)
 
 
@@ -445,9 +445,7 @@ class RequestContext:
         self.auto_pop(exc_value)
 
     def __repr__(self):
-        return "<{} '{}' [{}] of {}>".format(
-            self.__class__.__name__,
-            self.request.url,
-            self.request.method,
-            self.app.name,
+        return (
+            f"<{type(self).__name__} {self.request.url!r}"
+            f" [{self.request.method}] of {self.app.name}>"
         )

src/flask/debughelpers.py

@@ -29,17 +29,18 @@ class DebugFilesKeyError(KeyError, AssertionError):
     def __init__(self, request, key):
         form_matches = request.form.getlist(key)
         buf = [
-            'You tried to access the file "%s" in the request.files '
-            "dictionary but it does not exist.  The mimetype for the request "
-            'is "%s" instead of "multipart/form-data" which means that no '
-            "file contents were transmitted.  To fix this error you should "
-            'provide enctype="multipart/form-data" in your form.'
-            % (key, request.mimetype)
+            f"You tried to access the file {key!r} in the request.files"
+            " dictionary but it does not exist. The mimetype for the"
+            f" request is {request.mimetype!r} instead of"
+            " 'multipart/form-data' which means that no file contents"
+            " were transmitted. To fix this error you should provide"
+            ' enctype="multipart/form-data" in your form.'
         ]
         if form_matches:
+            names = ", ".join(repr(x) for x in form_matches)
             buf.append(
                 "\n\nThe browser instead transmitted some file names. "
-                "This was submitted: %s" % ", ".join('"%s"' % x for x in form_matches)
+                f"This was submitted: {names}"
             )
         self.msg = "".join(buf)
 
@@ -56,24 +57,24 @@ class FormDataRoutingRedirect(AssertionError):
     def __init__(self, request):
         exc = request.routing_exception
         buf = [
-            "A request was sent to this URL (%s) but a redirect was "
-            'issued automatically by the routing system to "%s".'
-            % (request.url, exc.new_url)
+            f"A request was sent to this URL ({request.url}) but a"
+            " redirect was issued automatically by the routing system"
+            f" to {exc.new_url!r}."
         ]
 
         # In case just a slash was appended we can be extra helpful
-        if request.base_url + "/" == exc.new_url.split("?")[0]:
+        if f"{request.base_url}/" == exc.new_url.split("?")[0]:
             buf.append(
-                "  The URL was defined with a trailing slash so "
-                "Flask will automatically redirect to the URL "
-                "with the trailing slash if it was accessed "
-                "without one."
+                "  The URL was defined with a trailing slash so Flask"
+                " will automatically redirect to the URL with the"
+                " trailing slash if it was accessed without one."
             )
 
         buf.append(
-            "  Make sure to directly send your %s-request to this URL "
-            "since we can't make browsers or HTTP clients redirect "
-            "with form data reliably or without user interaction." % request.method
+            "  Make sure to directly send your"
+            f" {request.method}-request to this URL since we can't make"
+            " browsers or HTTP clients redirect with form data reliably"
+            " or without user interaction."
         )
         buf.append("\n\nNote: this exception is only raised in debug mode")
         AssertionError.__init__(self, "".join(buf).encode("utf-8"))
@@ -101,16 +102,16 @@ def attach_enctype_error_multidict(request):
 
 
 def _dump_loader_info(loader):
-    yield "class: {}.{}".format(type(loader).__module__, type(loader).__name__)
+    yield f"class: {type(loader).__module__}.{type(loader).__name__}"
     for key, value in sorted(loader.__dict__.items()):
         if key.startswith("_"):
             continue
         if isinstance(value, (tuple, list)):
             if not all(isinstance(x, str) for x in value):
                 continue
-            yield "%s:" % key
+            yield f"{key}:"
             for item in value:
-                yield "  - %s" % item
+                yield f"  - {item}"
             continue
         elif not isinstance(value, (str, int, float, bool)):
             continue
@@ -119,7 +120,7 @@ def _dump_loader_info(loader):
 
 def explain_template_loading_attempts(app, template, attempts):
     """This should help developers understand what failed"""
-    info = ['Locating template "%s":' % template]
+    info = [f"Locating template {template!r}:"]
     total_found = 0
     blueprint = None
     reqctx = _request_ctx_stack.top
@@ -128,23 +129,23 @@ def explain_template_loading_attempts(app, template, attempts):
 
     for idx, (loader, srcobj, triple) in enumerate(attempts):
         if isinstance(srcobj, Flask):
-            src_info = 'application "%s"' % srcobj.import_name
+            src_info = f"application {srcobj.import_name!r}"
         elif isinstance(srcobj, Blueprint):
-            src_info = f'blueprint "{srcobj.name}" ({srcobj.import_name})'
+            src_info = f"blueprint {srcobj.name!r} ({srcobj.import_name})"
         else:
             src_info = repr(srcobj)
 
-        info.append("% 5d: trying loader of %s" % (idx + 1, src_info))
+        info.append(f"{idx + 1:5}: trying loader of {src_info}")
 
         for line in _dump_loader_info(loader):
-            info.append("       %s" % line)
+            info.append(f"       {line}")
 
         if triple is None:
             detail = "no match"
         else:
-            detail = "found (%r)" % (triple[1] or "<string>")
+            detail = f"found ({triple[1] or '<string>'!r})"
             total_found += 1
-        info.append("       -> %s" % detail)
+        info.append(f"       -> {detail}")
 
     seems_fishy = False
     if total_found == 0:
@@ -156,8 +157,8 @@ def explain_template_loading_attempts(app, template, attempts):
 
     if blueprint is not None and seems_fishy:
         info.append(
-            "  The template was looked up from an endpoint that "
-            'belongs to the blueprint "%s".' % blueprint
+            "  The template was looked up from an endpoint that belongs"
+            f" to the blueprint {blueprint!r}."
         )
         info.append("  Maybe you did not place a template in the right folder?")
         info.append("  See https://flask.palletsprojects.com/blueprints/#templates")
@@ -169,11 +170,10 @@ def explain_ignored_app_run():
     if os.environ.get("WERKZEUG_RUN_MAIN") != "true":
         warn(
             Warning(
-                "Silently ignoring app.run() because the "
-                "application is run from the flask command line "
-                "executable.  Consider putting app.run() behind an "
-                'if __name__ == "__main__" guard to silence this '
-                "warning."
+                "Silently ignoring app.run() because the application is"
+                " run from the flask command line executable. Consider"
+                ' putting app.run() behind an if __name__ == "__main__"'
+                " guard to silence this warning."
             ),
             stacklevel=3,
         )

src/flask/helpers.py

@@ -311,7 +311,7 @@ def url_for(endpoint, **values):
 
         if endpoint[:1] == ".":
             if blueprint_name is not None:
-                endpoint = blueprint_name + endpoint
+                endpoint = f"{blueprint_name}{endpoint}"
             else:
                 endpoint = endpoint[1:]
 
@@ -364,7 +364,7 @@ def url_for(endpoint, **values):
         return appctx.app.handle_url_build_error(error, endpoint, values)
 
     if anchor is not None:
-        rv += "#" + url_quote(anchor)
+        rv += f"#{url_quote(anchor)}"
     return rv
 
 
@@ -608,11 +608,12 @@ def send_file(
         try:
             attachment_filename = attachment_filename.encode("ascii")
         except UnicodeEncodeError:
+            quoted = url_quote(attachment_filename, safe="")
             filenames = {
                 "filename": unicodedata.normalize("NFKD", attachment_filename).encode(
                     "ascii", "ignore"
                 ),
-                "filename*": "UTF-8''%s" % url_quote(attachment_filename, safe=""),
+                "filename*": f"UTF-8''{quoted}",
             }
         else:
             filenames = {"filename": attachment_filename}
@@ -661,23 +662,19 @@ def send_file(
         from warnings import warn
 
         try:
-            rv.set_etag(
-                "%s-%s-%s"
-                % (
-                    os.path.getmtime(filename),
-                    os.path.getsize(filename),
-                    adler32(
-                        filename.encode("utf-8")
-                        if isinstance(filename, str)
-                        else filename
-                    )
-                    & 0xFFFFFFFF,
+            check = (
+                adler32(
+                    filename.encode("utf-8") if isinstance(filename, str) else filename
                 )
+                & 0xFFFFFFFF
+            )
+            rv.set_etag(
+                f"{os.path.getmtime(filename)}-{os.path.getsize(filename)}-{check}"
             )
         except OSError:
             warn(
-                "Access %s failed, maybe it does not exist, so ignore etags in "
-                "headers" % filename,
+                f"Access {filename} failed, maybe it does not exist, so"
+                " ignore etags in headers",
                 stacklevel=2,
             )
 
@@ -806,13 +803,12 @@ def get_root_path(import_name):
         # first module that is contained in our package.
         if filepath is None:
             raise RuntimeError(
-                "No root path can be found for the provided "
-                'module "%s".  This can happen because the '
-                "module came from an import hook that does "
-                "not provide file name information or because "
-                "it's a namespace package.  In this case "
-                "the root path needs to be explicitly "
-                "provided." % import_name
+                "No root path can be found for the provided module"
+                f" {import_name!r}. This can happen because the module"
+                " came from an import hook that does not provide file"
+                " name information or because it's a namespace package."
+                " In this case the root path needs to be explicitly"
+                " provided."
             )
 
     # filepath is import_name.py for a module, or __init__.py for a package.
@@ -823,6 +819,7 @@ def _matching_loader_thinks_module_is_package(loader, mod_name):
     """Given the loader that loaded a module and the module this function
     attempts to figure out if the given module is actually a package.
     """
+    cls = type(loader)
     # If the loader can tell us if something is a package, we can
     # directly ask the loader.
     if hasattr(loader, "is_package"):
@@ -830,20 +827,13 @@ def _matching_loader_thinks_module_is_package(loader, mod_name):
     # importlib's namespace loaders do not have this functionality but
     # all the modules it loads are packages, so we can take advantage of
     # this information.
-    elif (
-        loader.__class__.__module__ == "_frozen_importlib"
-        and loader.__class__.__name__ == "NamespaceLoader"
-    ):
+    elif cls.__module__ == "_frozen_importlib" and cls.__name__ == "NamespaceLoader":
         return True
     # Otherwise we need to fail with an error that explains what went
     # wrong.
     raise AttributeError(
-        (
-            "%s.is_package() method is missing but is required by Flask of "
-            "PEP 302 import hooks.  If you do not use import hooks and "
-            "you encounter this error please file a bug against Flask."
-        )
-        % loader.__class__.__name__
+        f"{cls.__name__}.is_package() method is missing but is required"
+        " for PEP 302 import hooks."
     )
 
 
@@ -1014,7 +1004,7 @@ class _PackageBoundObject:
 
         if self.static_folder is not None:
             basename = os.path.basename(self.static_folder)
-            return ("/" + basename).rstrip("/")
+            return f"/{basename}".rstrip("/")
 
     @static_url_path.setter
     def static_url_path(self, value):

src/flask/json/__init__.py

@@ -364,7 +364,7 @@ def jsonify(*args, **kwargs):
         data = args or kwargs
 
     return current_app.response_class(
-        dumps(data, indent=indent, separators=separators) + "\n",
+        f"{dumps(data, indent=indent, separators=separators)}\n",
         mimetype=current_app.config["JSONIFY_MIMETYPE"],
     )
 

src/flask/json/tag.py

@@ -105,7 +105,7 @@ class TagDict(JSONTag):
 
     def to_json(self, value):
         key = next(iter(value))
-        return {key + "__": self.serializer.tag(value[key])}
+        return {f"{key}__": self.serializer.tag(value[key])}
 
     def to_python(self, value):
         key = next(iter(value))

src/flask/sessions.py

@@ -208,13 +208,13 @@ class SessionInterface:
         rv = rv.rsplit(":", 1)[0].lstrip(".")
 
         if "." not in rv:
-            # Chrome doesn't allow names without a '.'
-            # this should only come up with localhost
-            # hack around this by not setting the name, and show a warning
+            # Chrome doesn't allow names without a '.'. This should only
+            # come up with localhost. Hack around this by not setting
+            # the name, and show a warning.
             warnings.warn(
-                '"{rv}" is not a valid cookie domain, it must contain a ".".'
-                " Add an entry to your hosts file, for example"
-                ' "{rv}.localdomain", and use that instead.'.format(rv=rv)
+                f"{rv!r} is not a valid cookie domain, it must contain"
+                " a '.'. Add an entry to your hosts file, for example"
+                f" '{rv}.localdomain', and use that instead."
             )
             app.config["SESSION_COOKIE_DOMAIN"] = False
             return None
@@ -232,7 +232,7 @@ class SessionInterface:
         # if this is not an ip and app is mounted at the root, allow subdomain
         # matching by adding a '.' prefix
         if self.get_cookie_path(app) == "/" and not ip:
-            rv = "." + rv
+            rv = f".{rv}"
 
         app.config["SESSION_COOKIE_DOMAIN"] = rv
         return rv

src/flask/testing.py

@@ -69,10 +69,9 @@ class EnvironBuilder(werkzeug.test.EnvironBuilder):
                 url_scheme = app.config["PREFERRED_URL_SCHEME"]
 
             url = url_parse(path)
-            base_url = "{scheme}://{netloc}/{path}".format(
-                scheme=url.scheme or url_scheme,
-                netloc=url.netloc or http_host,
-                path=app_root.lstrip("/"),
+            base_url = (
+                f"{url.scheme or url_scheme}://{url.netloc or http_host}"
+                f"/{app_root.lstrip('/')}"
             )
             path = url.path
 
@@ -114,7 +113,7 @@ class FlaskClient(Client):
         super().__init__(*args, **kwargs)
         self.environ_base = {
             "REMOTE_ADDR": "127.0.0.1",
-            "HTTP_USER_AGENT": "werkzeug/" + werkzeug.__version__,
+            "HTTP_USER_AGENT": f"werkzeug/{werkzeug.__version__}",
         }
 
     @contextmanager

src/flask/views.py

@@ -26,7 +26,7 @@ class View:
             methods = ['GET']
 
             def dispatch_request(self, name):
-                return 'Hello %s!' % name
+                return f"Hello {name}!"
 
         app.add_url_rule('/hello/<name>', view_func=MyView.as_view('myview'))
 
@@ -157,5 +157,5 @@ class MethodView(View, metaclass=MethodViewType):
         if meth is None and request.method == "HEAD":
             meth = getattr(self, "get", None)
 
-        assert meth is not None, "Unimplemented method %r" % request.method
+        assert meth is not None, f"Unimplemented method {request.method!r}"
         return meth(*args, **kwargs)

tests/conftest.py

@@ -116,9 +116,8 @@ def limit_loader(request, monkeypatch):
             self.loader = loader
 
         def __getattr__(self, name):
-            if name in ("archive", "get_filename"):
-                msg = "Mocking a loader which does not have `%s.`" % name
-                raise AttributeError(msg)
+            if name in {"archive", "get_filename"}:
+                raise AttributeError(f"Mocking a loader which does not have {name!r}.")
             return getattr(self.loader, name)
 
     old_get_loader = pkgutil.get_loader
@@ -148,7 +147,7 @@ def site_packages(modules_tmpdir, monkeypatch):
     """Create a fake site-packages."""
     rv = (
         modules_tmpdir.mkdir("lib")
-        .mkdir("python{x[0]}.{x[1]}".format(x=sys.version_info))
+        .mkdir(f"python{sys.version_info.major}.{sys.version_info.minor}")
         .mkdir("site-packages")
     )
     monkeypatch.syspath_prepend(str(rv))
@@ -161,23 +160,21 @@ def install_egg(modules_tmpdir, monkeypatch):
     sys.path."""
 
     def inner(name, base=modules_tmpdir):
-        if not isinstance(name, str):
-            raise ValueError(name)
         base.join(name).ensure_dir()
         base.join(name).join("__init__.py").ensure()
 
         egg_setup = base.join("setup.py")
         egg_setup.write(
             textwrap.dedent(
-                """
-        from setuptools import setup
-        setup(name='{}',
-              version='1.0',
-              packages=['site_egg'],
-              zip_safe=True)
-        """.format(
-                    name
+                f"""
+                from setuptools import setup
+                setup(
+                    name="{name}",
+                    version="1.0",
+                    packages=["site_egg"],
+                    zip_safe=True,
                 )
+                """
             )
         )
 

tests/test_basic.py

@@ -361,7 +361,7 @@ def test_session_localhost_warning(recwarn, app, client):
     rv = client.get("/", "http://localhost:5000/")
     assert "domain" not in rv.headers["set-cookie"].lower()
     w = recwarn.pop(UserWarning)
-    assert '"localhost" is not a valid cookie domain' in str(w.message)
+    assert "'localhost' is not a valid cookie domain" in str(w.message)
 
 
 def test_session_ip_warning(recwarn, app, client):
@@ -1095,7 +1095,7 @@ def test_enctype_debug_helper(app, client):
         with pytest.raises(DebugFilesKeyError) as e:
             client.post("/fail", data={"foo": "index.txt"})
         assert "no file contents were transmitted" in str(e.value)
-        assert 'This was submitted: "index.txt"' in str(e.value)
+        assert "This was submitted: 'index.txt'" in str(e.value)
 
 
 def test_response_types(app, client):
@@ -1821,7 +1821,7 @@ def test_subdomain_matching():
 
     @app.route("/", subdomain="<user>")
     def index(user):
-        return "index for %s" % user
+        return f"index for {user}"
 
     rv = client.get("/", "http://mitsuhiko.localhost.localdomain/")
     assert rv.data == b"index for mitsuhiko"
@@ -1834,7 +1834,7 @@ def test_subdomain_matching_with_ports():
 
     @app.route("/", subdomain="<user>")
     def index(user):
-        return "index for %s" % user
+        return f"index for {user}"
 
     rv = client.get("/", "http://mitsuhiko.localhost.localdomain:3000/")
     assert rv.data == b"index for mitsuhiko"

tests/test_blueprints.py

@@ -144,7 +144,7 @@ def test_blueprint_url_defaults(app, client):
 
     @bp.route("/foo", defaults={"baz": 42})
     def foo(bar, baz):
-        return "%s/%d" % (bar, baz)
+        return f"{bar}/{baz:d}"
 
     @bp.route("/bar")
     def bar(bar):

tests/test_cli.py

@@ -268,9 +268,9 @@ def test_get_version(test_apps, capsys):
     ctx = MockCtx()
     get_version(ctx, None, "test")
     out, err = capsys.readouterr()
-    assert "Python " + python_version() in out
-    assert "Flask " + flask_version in out
-    assert "Werkzeug " + werkzeug_version in out
+    assert f"Python {python_version()}" in out
+    assert f"Flask {flask_version}" in out
+    assert f"Werkzeug {werkzeug_version}" in out
 
 
 def test_scriptinfo(test_apps, monkeypatch):
@@ -288,7 +288,7 @@ def test_scriptinfo(test_apps, monkeypatch):
     app = obj.load_app()
     assert app.name == "testapp"
     assert obj.load_app() is app
-    obj = ScriptInfo(app_import_path=cli_app_path + ":testapp")
+    obj = ScriptInfo(app_import_path=f"{cli_app_path}:testapp")
     app = obj.load_app()
     assert app.name == "testapp"
     assert obj.load_app() is app
@@ -406,7 +406,7 @@ def test_flaskgroup_debug(runner, set_debug_flag):
 
     result = runner.invoke(cli, ["test"])
     assert result.exit_code == 0
-    assert result.output == "%s\n" % str(not set_debug_flag)
+    assert result.output == f"{not set_debug_flag}\n"
 
 
 def test_print_exceptions(runner):
@@ -656,4 +656,4 @@ def test_cli_empty(app):
     app.register_blueprint(bp)
 
     result = app.test_cli_runner().invoke(args=["blue", "--help"])
-    assert result.exit_code == 2, "Unexpected success:\n\n" + result.output
+    assert result.exit_code == 2, f"Unexpected success:\n\n{result.output}"

tests/test_config.py

@@ -28,7 +28,7 @@ def common_object_test(app):
 
 def test_config_from_pyfile():
     app = flask.Flask(__name__)
-    app.config.from_pyfile(__file__.rsplit(".", 1)[0] + ".py")
+    app.config.from_pyfile(f"{__file__.rsplit('.', 1)[0]}.py")
     common_object_test(app)
 
 
@@ -84,7 +84,7 @@ def test_config_from_envvar(monkeypatch):
     assert not app.config.from_envvar("FOO_SETTINGS", silent=True)
 
     monkeypatch.setattr(
-        "os.environ", {"FOO_SETTINGS": __file__.rsplit(".", 1)[0] + ".py"}
+        "os.environ", {"FOO_SETTINGS": f"{__file__.rsplit('.', 1)[0]}.py"}
     )
     assert app.config.from_envvar("FOO_SETTINGS")
     common_object_test(app)
@@ -185,12 +185,10 @@ def test_from_pyfile_weird_encoding(tmpdir, encoding):
     f = tmpdir.join("my_config.py")
     f.write_binary(
         textwrap.dedent(
+            f"""
+            # -*- coding: {encoding} -*-
+            TEST_VALUE = "föö"
             """
-    # -*- coding: {} -*-
-    TEST_VALUE = "föö"
-    """.format(
-                encoding
-            )
         ).encode(encoding)
     )
     app = flask.Flask(__name__)

tests/test_helpers.py

@@ -286,7 +286,7 @@ class TestJSON:
         class MyEncoder(flask.json.JSONEncoder):
             def default(self, o):
                 if isinstance(o, X):
-                    return "<%d>" % o.val
+                    return f"<{o.val}>"
                 return flask.json.JSONEncoder.default(self, o)
 
         class MyDecoder(flask.json.JSONDecoder):
@@ -314,14 +314,16 @@ class TestJSON:
         assert rv.data == b'"<42>"'
 
     def test_blueprint_json_customization(self, app, client):
-        class X:  # noqa: B903, for Python2 compatibility
+        class X:
+            __slots__ = ("val",)
+
             def __init__(self, val):
                 self.val = val
 
         class MyEncoder(flask.json.JSONEncoder):
             def default(self, o):
                 if isinstance(o, X):
-                    return "<%d>" % o.val
+                    return f"<{o.val}>"
 
                 return flask.json.JSONEncoder.default(self, o)
 
@@ -687,9 +689,9 @@ class TestSendfile:
         )
         rv.close()
         content_disposition = rv.headers["Content-Disposition"]
-        assert "filename=%s" % ascii in content_disposition
+        assert f"filename={ascii}" in content_disposition
         if utf8:
-            assert "filename*=UTF-8''" + utf8 in content_disposition
+            assert f"filename*=UTF-8''{utf8}" in content_disposition
         else:
             assert "filename*=UTF-8''" not in content_disposition
 
@@ -818,7 +820,7 @@ class TestUrlFor:
             def get(self, id=None):
                 if id is None:
                     return "List"
-                return "Get %d" % id
+                return f"Get {id:d}"
 
             def post(self):
                 return "Create"

tests/test_reqctx.py

@@ -110,7 +110,7 @@ def test_proper_test_request_context(app):
 def test_context_binding(app):
     @app.route("/")
     def index():
-        return "Hello %s!" % flask.request.args["name"]
+        return f"Hello {flask.request.args['name']}!"
 
     @app.route("/meh")
     def meh():
@@ -138,7 +138,7 @@ def test_context_test(app):
 def test_manual_context_binding(app):
     @app.route("/")
     def index():
-        return "Hello %s!" % flask.request.args["name"]
+        return f"Hello {flask.request.args['name']}!"
 
     ctx = app.test_request_context("/?name=World")
     ctx.push()

tests/test_templating.py

@@ -414,16 +414,16 @@ def test_template_loader_debugging(test_apps, monkeypatch):
         def handle(self, record):
             called.append(True)
             text = str(record.msg)
-            assert '1: trying loader of application "blueprintapp"' in text
+            assert "1: trying loader of application 'blueprintapp'" in text
             assert (
-                '2: trying loader of blueprint "admin" (blueprintapp.apps.admin)'
+                "2: trying loader of blueprint 'admin' (blueprintapp.apps.admin)"
             ) in text
             assert (
-                'trying loader of blueprint "frontend" (blueprintapp.apps.frontend)'
+                "trying loader of blueprint 'frontend' (blueprintapp.apps.frontend)"
             ) in text
             assert "Error: the template could not be found" in text
             assert (
-                'looked up from an endpoint that belongs to the blueprint "frontend"'
+                "looked up from an endpoint that belongs to the blueprint 'frontend'"
             ) in text
             assert "See https://flask.palletsprojects.com/blueprints/#templates" in text
 

tests/test_testing.py

@@ -59,7 +59,7 @@ def test_environ_base_default(app, client, app_ctx):
 
     rv = client.get("/")
     assert rv.data == b"127.0.0.1"
-    assert flask.g.user_agent == "werkzeug/" + werkzeug.__version__
+    assert flask.g.user_agent == f"werkzeug/{werkzeug.__version__}"
 
 
 def test_environ_base_modified(app, client, app_ctx):
@@ -321,7 +321,7 @@ def test_json_request_and_response(app, client):
 def test_client_json_no_app_context(app, client):
     @app.route("/hello", methods=["POST"])
     def hello():
-        return "Hello, {}!".format(flask.request.json["name"])
+        return f"Hello, {flask.request.json['name']}!"
 
     class Namespace:
         count = 0

tests/test_user_error_handler.py

@@ -29,7 +29,7 @@ def test_error_handler_no_match(app, client):
         original = getattr(e, "original_exception", None)
 
         if original is not None:
-            return "wrapped " + type(original).__name__
+            return f"wrapped {type(original).__name__}"
 
         return "direct"
 
@@ -238,9 +238,9 @@ class TestGenericHandlers:
         original = getattr(e, "original_exception", None)
 
         if original is not None:
-            return "wrapped " + type(original).__name__
+            return f"wrapped {type(original).__name__}"
 
-        return "direct " + type(e).__name__
+        return f"direct {type(e).__name__}"
 
     @pytest.mark.parametrize("to_handle", (InternalServerError, 500))
     def test_handle_class_or_code(self, app, client, to_handle):