Various utility functions shipped with Werkzeug.

HTML Helpers

class werkzeug.utils.HTMLBuilder(dialect)

Helper object for HTML generation.

Per default there are two instances of that class. The html one, and the xhtml one for those two dialects. The class uses keyword parameters and positional parameters to generate small snippets of HTML.

Keyword parameters are converted to XML/SGML attributes, positional arguments are used as children. Because Python accepts positional arguments before keyword arguments it’s a good idea to use a list with the star-syntax for some children:

>>> html.p(class_='foo', *[html.a('foo', href='foo.html'), ' ',
...                        html.a('bar', href='bar.html')])
u'<p class="foo"><a href="foo.html">foo</a> <a href="bar.html">bar</a></p>'

This class works around some browser limitations and can not be used for arbitrary SGML/XML generation. For that purpose lxml and similar libraries exist.

Calling the builder escapes the string passed:

>>> html.p(html("<foo>"))

Replace special characters “&”, “<”, “>” and (“) to HTML-safe sequences.

There is a special handling for None which escapes to an empty string.


Changed in version 0.9: quote is now implicitly on.

  • s – the string to escape.
  • quote – ignored.

The reverse function of escape. This unescapes all the HTML entities, not only the XML entities inserted by escape.

Parameters: s – the string to unescape.

General Helpers

class werkzeug.utils.cached_property(func, name=None, doc=None)

A decorator that converts a function into a lazy property. The function wrapped is called the first time to retrieve the result and then that calculated result is used the next time you access the value:

class Foo(object):

    def foo(self):
        # calculate something important here
        return 42

The class has to have a __dict__ in order for this property to work.

werkzeug.utils.invalidate_cached_property(obj, name)

Invalidates the cache for a cached_property:

>>> class Test(object):
...     @cached_property
...     def magic_number(self):
...         print("recalculating...")
...         return 42
>>> var = Test()
>>> var.magic_number
>>> var.magic_number
>>> invalidate_cached_property(var, "magic_number")
>>> var.magic_number

You must pass the name of the cached property as the second argument.

class werkzeug.utils.environ_property(name, default=None, load_func=None, dump_func=None, read_only=None, doc=None)

Maps request attributes to environment variables. This works not only for the Werzeug request object, but also any other class with an environ attribute:

>>> class Test(object):
...     environ = {'key': 'value'}
...     test = environ_property('key')
>>> var = Test()
>>> var.test

If you pass it a second value it’s used as default if the key does not exist, the third one can be a converter that takes a value and converts it. If it raises ValueError or TypeError the default value is used. If no default value is provided None is used.

Per default the property is read only. You have to explicitly enable it by passing read_only=False to the constructor.

class werkzeug.utils.header_property(name, default=None, load_func=None, dump_func=None, read_only=None, doc=None)

Like environ_property but for headers.

werkzeug.utils.redirect(location, code=302, Response=None)

Returns a response object (a WSGI application) that, if called, redirects the client to the target location. Supported codes are 301, 302, 303, 305, 307, and 308. 300 is not supported because it’s not a real redirect and 304 because it’s the answer for a request with a request with defined If-Modified-Since headers.


New in version 0.10: The class used for the Response object can now be passed in.

New in version 0.6: The location can now be a unicode string that is encoded using the iri_to_uri() function.

  • location – the location the response should redirect to.
  • code – the redirect status code. defaults to 302.
  • Response (class) – a Response class to use when instantiating a response. The default is werkzeug.wrappers.Response if unspecified.
werkzeug.utils.append_slash_redirect(environ, code=301)

Redirects to the same URL but with a slash appended. The behavior of this function is undefined if the path ends with a slash already.

  • environ – the WSGI environment for the request that triggers the redirect.
  • code – the status code for the redirect.
werkzeug.utils.import_string(import_name, silent=False)

Imports an object based on a string. This is useful if you want to use import paths as endpoints or something similar. An import path can be specified either in dotted notation (xml.sax.saxutils.escape) or with a colon as object delimiter (xml.sax.saxutils:escape).

If silent is True the return value will be None if the import fails.

  • import_name – the dotted name for the object to import.
  • silent – if set to True import errors are ignored and None is returned instead.

imported object

werkzeug.utils.find_modules(import_path, include_packages=False, recursive=False)

Finds all the modules below a package. This can be useful to automatically import all views / controllers so that their metaclasses / function decorators have a chance to register themselves on the application.

Packages are not returned unless include_packages is True. This can also recursively list modules but in that case it will import all the packages to get the correct load path of that module.

  • import_path – the dotted name for the package to find child modules.
  • include_packages – set to True if packages should be returned, too.
  • recursive – set to True if recursion should happen.


werkzeug.utils.validate_arguments(func, args, kwargs, drop_extra=True)

Checks if the function accepts the arguments and keyword arguments. Returns a new (args, kwargs) tuple that can safely be passed to the function without causing a TypeError because the function signature is incompatible. If drop_extra is set to True (which is the default) any extra positional or keyword arguments are dropped automatically.

The exception raised provides three attributes:

A set of argument names that the function expected but where missing.
A dict of keyword arguments that the function can not handle but where provided.
A list of values that where given by positional argument but the function cannot accept.

This can be useful for decorators that forward user submitted data to a view function:

from werkzeug.utils import ArgumentValidationError, validate_arguments

def sanitize(f):
    def proxy(request):
        data = request.values.to_dict()
            args, kwargs = validate_arguments(f, (request,), data)
        except ArgumentValidationError:
            raise BadRequest('The browser failed to transmit all '
                             'the data expected.')
        return f(*args, **kwargs)
    return proxy
  • func – the function the validation is performed against.
  • args – a tuple of positional arguments.
  • kwargs – a dict of keyword arguments.
  • drop_extra – set to False if you don’t want extra arguments to be silently dropped.

tuple in the form (args, kwargs).


Pass it a filename and it will return a secure version of it. This filename can then safely be stored on a regular file system and passed to os.path.join(). The filename returned is an ASCII only string for maximum portability.

On windows systems the function also makes sure that the file is not named after one of the special device files.

>>> secure_filename("My cool movie.mov")
>>> secure_filename("../../../etc/passwd")
>>> secure_filename(u'i contain cool \xfcml\xe4uts.txt')

The function might return an empty filename. It’s your responsibility to ensure that the filename is unique and that you abort or generate a random filename if the function returned an empty one.


New in version 0.5.

Parameters: filename – the filename to secure
werkzeug.utils.bind_arguments(func, args, kwargs)

Bind the arguments provided into a dict. When passed a function, a tuple of arguments and a dict of keyword arguments bind_arguments returns a dict of names as the function would see it. This can be useful to implement a cache decorator that uses the function arguments to build the cache key based on the values of the arguments.

  • func – the function the arguments should be bound for.
  • args – tuple of positional arguments.
  • kwargs – a dict of keyword arguments.

a dict of bound keyword arguments.

URL Helpers

Please refer to URL Helpers.

UserAgent Parsing

class werkzeug.useragents.UserAgent(environ_or_string)

Represents a user agent. Pass it a WSGI environment or a user agent string and you can inspect some of the details from the user agent string via the attributes. The following attributes exist:


the raw user agent string


the browser platform. None if not recognized. The following platforms are currently recognized:

  • aix
  • amiga
  • android
  • blackberry
  • bsd
  • chromeos
  • dragonflybsd
  • freebsd
  • hpux
  • ipad
  • iphone
  • irix
  • linux
  • macos
  • netbsd
  • openbsd
  • sco
  • solaris
  • symbian
  • wii
  • windows

the name of the browser. None if not recognized. The following browsers are currently recognized:

  • aol *
  • ask *
  • baidu *
  • bing *
  • camino
  • chrome
  • edge
  • firefox
  • galeon
  • google *
  • kmeleon
  • konqueror
  • links
  • lynx
  • mozilla
  • msie
  • msn
  • netscape
  • opera
  • safari
  • seamonkey
  • webkit
  • yahoo *

(Browsers marked with a star (*) are crawlers.)


the version of the browser. None if not recognized.


the language of the browser. None if not recognized.

Security Helpers


New in version 0.6.1.

werkzeug.security.generate_password_hash(password, method='pbkdf2:sha256', salt_length=8)

Hash a password with the given method and salt with a string of the given length. The format of the string returned includes the method that was used so that check_password_hash() can check the hash.

The format for the hashed string looks like this:


This method can not generate unsalted passwords but it is possible to set param method=’plain’ in order to enforce plaintext passwords. If a salt is used, hmac is used internally to salt the password.

If PBKDF2 is wanted it can be enabled by setting the method to pbkdf2:method:iterations where iterations is optional:

  • password – the password to hash.
  • method – the hash method to use (one that hashlib supports). Can optionally be in the format pbkdf2:<method>[:iterations] to enable PBKDF2.
  • salt_length – the length of the salt in letters.
werkzeug.security.check_password_hash(pwhash, password)

check a password against a given salted and hashed password value. In order to support unsalted legacy passwords this method supports plain text passwords, md5 and sha1 hashes (both salted and unsalted).

Returns True if the password matched, False otherwise.

  • pwhash – a hashed string like returned by generate_password_hash().
  • password – the plaintext password to compare against the hash.
werkzeug.security.safe_str_cmp(a, b)

This function compares strings in somewhat constant time. This requires that the length of at least one string is known in advance.

Returns True if the two strings are equal, or False if they are not.


New in version 0.7.

werkzeug.security.safe_join(directory, *pathnames)

Safely join zero or more untrusted path components to a base directory to avoid escaping the base directory.

  • directory – The trusted base directory.
  • pathnames – The untrusted path components relative to the base directory.

A safe path, otherwise None.

werkzeug.security.pbkdf2_hex(data, salt, iterations=150000, keylen=None, hashfunc=None)

Like pbkdf2_bin(), but returns a hex-encoded string.


New in version 0.9.

  • data – the data to derive.
  • salt – the salt for the derivation.
  • iterations – the number of iterations.
  • keylen – the length of the resulting key. If not provided, the digest size will be used.
  • hashfunc – the hash function to use. This can either be the string name of a known hash function, or a function from the hashlib module. Defaults to sha256.
werkzeug.security.pbkdf2_bin(data, salt, iterations=150000, keylen=None, hashfunc=None)

Returns a binary digest for the PBKDF2 hash algorithm of data with the given salt. It iterates iterations times and produces a key of keylen bytes. By default, SHA-256 is used as hash function; a different hashlib hashfunc can be provided.


New in version 0.9.

  • data – the data to derive.
  • salt – the salt for the derivation.
  • iterations – the number of iterations.
  • keylen – the length of the resulting key. If not provided the digest size will be used.
  • hashfunc – the hash function to use. This can either be the string name of a known hash function or a function from the hashlib module. Defaults to sha256.


Werkzeug uses standard Python logging. The logger is named "werkzeug".

import logging
logger = logging.getLogger("werkzeug")

If the logger level is not set, it will be set to INFO on first use. If there is no handler for that level, a StreamHandler is added.

© 2007–2020 Pallets
Licensed under the BSD 3-clause License.