Writing Profiles

A Font Bakery Profile (an instance of the type fontbakery.checkrunner.Profile) is a container for a set of checks to be run on font files.

A profile usually lives inside a Python module: one module contains one profile. Font Bakery comes with a number of profiles, located in fontbakery.profiles.*, which can be used directly, but you can also create a custom profile. A custom profile can include your own checks, written in Python, and/or checks included from other profiles. Writing a custom Font Bakery profile can be a good way to either ensure the quality of a single font project, or for a font foundry or maintainer of a font library to establish comprehensive quality standards and quality monitoring.

Font Bakery comes with a set of checks that can be included into such a custom profile according to the requirements of the author. These checks are mainly organized in profile modules named after OpenType tables, and can be found at fontbakery.profiles.*.

When you decide to include a profile or a single check from a Font Bakery profile into your own custom profile, you’ll end up reading and reviewing the profile’s Python code. We welcome questions, remarks, improvements, additions, more documentation and other contributions. Please use our issue tracker to contact us or send pull requests.

fontbakery.profiles.googlefonts is a custom profile and can be an interesting piece of code to inspect for some inspiration.

From automatic discovery to full control

To create your own profile we added tools that reduce boilerplate code to a minimum. These tools are fully optional. But it is possible to use them if you need more control over the creation of a profile. This automatic discovery can be enabled or disabled based on the presence (and value) of some special names in the module of the profile:

  • profile

  • profile_factory

  • profile_imports

To get or create a profile from a module, the function get_module_profile(module) in fontbakery.checkrunner is used:

If the name module.profile is present the value of that is returned. Otherwise, if the name module.profile_factory is present, a new profile is created using module.profile_factory and then profile.auto_register is called with the module namespace.

Thus, the presence of the name profile disables the automatic use of profile_factory plus profile.auto_register. But you can consider calling profile.auto_register(globals()) explicitly yourself near the end of your profile module or you can register the profile contents yourself.

# profile = Profile(...)
# globals is a Python builtin

profile.auto_register(symbol_table, filter_func=None)

Whether you call auto_register yourself or if it is called automatically for you, it is important to understand how it operates: It looks at all values of its symbol_table argument (a namespace dict as returned from globals() or module.__dict__) and at all values imported using profile_imports (described below) if present in symbol_table.

If one of the items is an instance of one of FontBakeryCheck, FontBakeryCondition, FontBakeryExpectedValue the respective (public) interface methods: profile.register_check, profile.profile and profile.register_expected_value are called.

If an item is a python module (an instance of types.ModuleType) it is tried to get a profile from that module using sub_profile = get_module_profile(item). This, in many cases, invokes get_module_profile recursively, which itself calls auto_register when indicated (see above). That new sub-profile is then included into the current auto-registering profile using profile.merge_profile(sub_profile) (see below).

When you are calling profile.auto_register explicitly yourself, you can also use the filter_func argument. This gives you finer control over which items are loaded into your profile, you can use it to implement blacklist and/or whitelist filtering.

Here is an example where filter_func was needed due to a namespace clash.

If a filter_func argument is defined it is called like filter_func(type, name_or_id, item) where:

  • type is one of "check", "module", "condition", "expected_value", "iterarg", "derived_iterable", "alias"

  • name_or_id is the name at which the item will be registered

    • if type == 'check': the check.id

    • if type == 'module': the module name (module.__name__)

  • item is the item to be registered

If filter_func returns a falsy value for an item, the item will not be registered.

profile_imports used by profile.auto_register

If profile_imports is defined in a module namespace, profile.auto_register will use it to import modules and attributes of a module into the profile.

You could also use just the regular python import statement, which puts imports into the current module namespace and by that makes them discoverable for profile.auto_register. However, with that approach static linters like flake8 and pylint will complain about unused imports in your profile. That’s why we added profile_imports.

profile_imports must be a list or tuple of import instructions, mimicking the import module_name and from module_name import (identifier, ...) syntax. Disclaimer: It could be more compatible to the python import statements, but the algorithm of python is rather complicated (feel free to contribute).

If an import instruction entry in profile_imports is a string: just try to import that module. If an import instruction is not a string it is expected to be a two items iterable (tuple or list): (module_name, (identifier, ...)).

  • If module_name consists of only “dots” (.) all identifers are imported as module names relative from the current module.

  • Otherwise all identifiers are treated as attributes of the module with module_name, which also can be a relative import when using leading dots.

NOTE: one common pitfall is the python syntax for tuple literals. If a tuple has just one item, it still needs a comma after that item to make it a tuple and not just parentheses i.e. ("my_identifier", ). For that reason the following examples use lists, although tuples, being immutable, are semantically a better fit.

# profile_imports examples

# Import relative to the current module the
# modules shared_conditions and ufo_sources.
# This includes all of the shared_conditions
# and ufo_sources profile into the
# current profile:
profile_imports = [".shared_conditions", ".ufo_sources"]

# The following is equivalent to the previous statement:
profile_imports = [
    [".", ["shared_conditions", "ufo_sources"]]

# Also same as above:
profile_imports = [
    [".", [ "ufo_sources"]]
# Here's an example copied from fontbakery.profiles.googlefonts
# This includes all of our Open Type profiles and some more into
# the googlefonts profile:
profile_imports = (
    ('.', ('general', 'cmap', 'head', 'os2', 'post', 'name',
       'hhea', 'dsig', 'hmtx', 'gpos', 'gdef', 'kern', 'glyf',
       'prep', 'fvar', 'shared_conditions')
    ), # IMPORTANT: this must be a tuple, note the trailing comma
# Import just certain attributes from modules.
# Also, using absolute import module names:
profile_imports = [
    # like we do in fontbakery.profiles.fvar
    ('fontbakery.profiles.shared_conditions', ('is_variable_font',
            'regular_wght_coord', 'regular_wdth_coord', 'regular_slnt_coord',
            'regular_ital_coord', 'regular_opsz_coord', 'bold_wght_coord')),
    # just as an example: import a check and a dependency/condition of
    # that check from the googlefonts specific profile:
    ('fontbakery.profiles.googlefonts', (
        # "License URL matches License text on name table?"
        # This condition is a dependency of the check above:

profile.merge_profile(other_profile, filter_func=None)

Copy all namespace items from other_profile to profile.

Namespace items are: iterargs, derived_iterables, aliases, conditions, expected_values.

Don’t change any contents of other_profile, it may modify the loaded python module! That means sections are cloned not used directly.

Optional argument filter_func: see description in auto_register.

profile.test_expected_checks(expected_check_ids, exclusive=True)

Self-test to make a sure profile maintainer is aware of changes in the profile.

Raises SetupError if expected check ids are missing in the profile, e.g. removed by an update. If exclusive=True also raises SetupError if check ids are in the profile that are not in expected_check_ids e.g. newly added by an update.

This is handy if profile.auto_register is used and the profile maintainer is looking for a high level of control over the profile contents, especially for a warning when the profile contents have changed after an update of a dependency profile but it also makes profile maintenance a bit more laborious.


An ordered set of checks with a name.

Used to structure checks in a profile. A profile consists of one or more sections.

Running profiles

$ fontbakery check-profile fontbakery ./path/to/fonts/*

Writing Profiles: Quick start

A very basic profile with no apparent use but to document profiles, checks and conditions. For real world use cases look into the modules at fontbakery.profiles.*


# The example and other custom profiles can be executed with this command:
$ fontbakery check-profile ./path/to/my/example_profile.py ./path/to/fonts/*

# But if you have installed your profile as a package, then use:
$ fontbakery check-profile my_package.example_profile ./path/to/fonts/*

Commented line by line:

# We are going to define checks and conditons
from fontbakery.callable import check, condition
# All possible statuses a check can yield, in order of
# severity. The least severe being DEBUG. The most severe
# status emitted by a check is the end result of that check.
# DEBUG can't be an end result, the least severe status
# allowed as a check is PASS.
from fontbakery.checkrunner import (DEBUG, PASS,
               INFO, SKIP, WARN, FAIL, ERROR)
# Used to inform get_module_profile whether and
# how to create a profile. This
# example will create an instance of `FontsProfile`.
# The comment at the end of the line disables flake8
# and pylint to complain about unused imports.
from fontbakery.fonts_profile import profile_factory # NOQA pylint: disable=unused-import

# At this point we already have a importable profile
# It needs some checks though, to be useful.

# profile_imports can be used to mix other profiles
# into this profile. We are only using two profiles
# for this example, containing checks for the accordingly
# named tables
profile_imports = [
    ['fontbakery.profiles', ['cmap', 'head']]

# Now we picked some checks from other profiles, but
# what about defining checks ourselves.

# We use `check` as a decorator to wrap an ordinary python
# function into an instance of FontBakeryCheck to prepare
# and mark it as a check.
# A check id is mandatory and must be globally and timely
# unique. See "Naming Things: check-ids" below.
# This check will run only once as it has no iterable
# arguments. Since it has no arguments at all and because
# checks should be idempotent (and this one is), there's
# not much sense in having it all. It will run once
# and always yield the same result.
def hello_world():
  """Simple "Hello World" example."""
  # The function name of a check is not very important
  # to create it, only to import it from another module
  # or to call it directly, However, a short line of
  # human readable description is mandatory, preferable
  # via the docstring of the check.
  # A status of a check can be `return`ed or `yield`ed
  # depending on the nature of the check, `return`
  # can only return just one status while `yield`
  # makes a generator out of it and it can produce
  # many statuses.
  # A status also always must be a tuple of (Status, Message)
  # For `Message` a string is OK, but for unit testing
  # it turned out that an instance of `fontbakery.message.Message`
  # can be very useful. It can additionally provide
  # a status code, better suited to figure out the exact
  # check result.
  yield PASS, 'Hello World'

# This check will run once for each item in `fonts`.
# This is achieved via the iterag definition of font: fonts
def has_cap_r_in_name(font):
  """Filename contains an "R"."""
  # This check is not very useful again, but for each
  # input it can result in a PASS or a FAIL.
  if 'R' not in font:
    # This is our first check that can potentially fail.
    # To document this: return is also ok in a check.
    return FAIL, '"R" is not in font filename.'
    # since you can't return at one point in a function
    # and yield at another point, we always have to
    # use return within this check.
    return PASS, '"R" is in font filename.'

# conditions are used for the dependency injection as arguments
# and to decide if a check will be skipped
# ttFont is a condition built into FontProfile
# it returns an instance of fontTools.TTLib.TTFont
def is_ttf(ttFont):
   return 'glyf' in ttFont
    # this check will be skipped if the font is not a ttf
# this also runs once per font in fonts, but its called with
# the ttFont instance
def has_ttf_glyphs(ttFont):
  """ It's bad when there are no glyphs in the TTF."""
  # savely use the "glyf" table, because of conditions="is_ttf"
  # we know it's available
  if not len(ttFont['glyf'].glyphs):
    return FAIL, "There are no glyphs in this TTF."
  return PASS, "Some gLyphs are in this TTF."

Now to disable the automatic creation and registration while creating an equivalent result:

# maybe at the top of the file
from fontbakery.checkrunner import Section

# you can use a another name for the default section
# but this is what get_module_profile would do
profile = profile_factory(default_section=Section(__name__))
# IMPORTANT: after all checks etc. have ben defined:

Since we have the profile reference we can also use the expected_check_ids self check method.

# putting this at the top of the file
# can give a guick overview:
expected_check_ids = (
# this must be at the end of the module,
# after all checks were added:
profile.test_expected_checks(expected_check_ids, exclusive=True)

You can also explicitly import and merge a profile:

from fontbakery.profiles import os2
from fontbakery.checkrunner import get_module_profile
# using get_module_profile is the recommended way to get a
# profile from a module, because it takes care of creating it
# automatically if not explicitly defined.
os2_profile = get_module_profile(os2)

Naming Things: check-ids

Check IDs should ideally be globally and temporally unique. This means we once assign them and we never change them again. We plan to start enforcing this after FontBakery version 1.0.0 is released. This can then be used to keep track of the history of checks and check-results. We already rely on this feature for our tooling and we’ll rely more on it in the future. within a profile a check id must be unique as well, to make sharing of checks and mixing of profiles easier. Globally unique names are a good thing.

We want to encourage authors to contribute their own checks to the Font Bakery collection, if they are generally useful and fit into it. Therefore an agreement on how to create check ids is needed to avoid id-collisions.

Reverse domain name notation has proven useful and a nice side effect is that contributors to Font Bakery will stay appreciated. Here are some examples:

com.daltonmaag/check/required-fields and de.graphicore.fontbakery/profile-examples/hello: both examples are valid and more or less descriptive.

Of course, after your personal prefix, organizing names is up to you. One proposal is e.g. for checks that are specific to certain tables, the table name could be used as part of the check id.

Naming Things: The Dependency Injection Namespace

IMPORTANT: Names can and will clash e.g. when sharing between profiles. This is a good thing, because we need to be sure the right definitions are used for our profile. On the other hand, namespace hygiene will stay an ongoing topic.

The dependency injection of the Font Bakery Check Runner is a feature that makes it easy to write actual check implementations.

Example: Run a check once for each fontTools.ttLib.TTFont instance of all fonts: Just use ttFont as a parameter name.

# example from fontbakery.profiles.post
def com_google_fonts_check_post_table_version(ttFont):
  """Font has post table version 2?"""
  if ttFont['post'].formatType != 2:
    yield FAIL, ("Post table should be version 2 instead of {}."
                 " More info at https://github.com/google/fonts/"
    yield PASS, "Font has post table version 2."

To achieve this, Font Bakery uses inspection (meta programming) to get the parameter names of a check (or condition) and then resolves these dependencies using the matching definitions in the profile namespace.

There’s a collection of different types that together populate the profile namespace. We need almost all of them to make our example from above work. Yet in this case they are already predefined in FontsProfile, making this also a documentation of those.

fonts Expected Value

FontProfile also defines a command line argument fonts, which is then passed as a value to the check runner.

In order to to make the namespace aware of a value that’s going to be provided when the check is executed, a FontBakeryExpectedValue is defined and added to the profile.

from fontbakery.callable import FontBakeryExpectedValue
fonts_expected_value = FontBakeryExpectedValue(
    , default=[]
    , description='A list of the font file paths to check.'
    , validator=lambda fonts: (True, None) if len(fonts) \
                                    else (False, 'Value is empty.')
# somehow added to profile, e.g.:
# profile.register_expected_value(fonts_expected_value)

fonts when used as a check argument will be something like:

['Myfont-Regular.ttf', 'Myfont-Italic.ttf', ...]

font Iterarg

Iterargs (iterable arguments) are defined via the Profile constructor.

profile = FontsProfile(
      iterargs={'font': 'fonts'},

This tells Font Bakery when we use font as a check parameter, to call the check once for each font in fonts.

def once_per_font(font):

# Will be executed something along these lines:

ttFont Condition

Conditions are somehow similar to checks: they are also functions and they are also using the same dependency injection mechanism like checks for their parameters. But they are also part of the namespace and dependency injection.

def ttFont(font):
   from fontTools.ttLib import TTFont
   return TTFont(font)

# somehow added to profile, e.g.:
# profile.register_condition(ttFont)

This, when used in a check as parameter, will inherit from the font iterarg, that the check will be executed once for each font in fonts. However, the argument passed will be an instance of TTFont:

def once_per_ttfont(ttFont):

# will be called like:
once_per_font( ttFont(...))

ttFonts Derived Iterables

Derived Iterables are not used in the example above, they however make it possible for a condition like ttFont to create an iterator for all of it values:

profile = FontsProfile(
      derived_iterables={'ttFonts': ('ttFont', True)}

Makes it possible to:

def once_for_all_ttfonts(ttFonts):

# is called much like:
once_for_all_ttfonts([ttFont(font) for font in [
                  'Myfont-Regular.ttf', 'Myfont-Italic.ttf', ...]])


Not used at the moment but meant to provide a mechanism to map existing names to new names. Maybe this feature will disappear again when we don’t find a use case for it.

profile = FontsProfile(
      aliases={'new_name': 'old_name'},

Explicit Namespace Overrides force=True

In order to make it possible to create checks and conditions that depend on values unknown in advance, it is possible to forcefully override certain namespace entries with a later definition. For this FontBakeryCondition and FontBakeryExpected value have an force=True optional parameter and for the other namespace types profile.add_to_namespace can be used directly, which provides the same parameter. Just don’t over do it and take care of namspace clashes carefully!