Markers

One extra requirement of dealing with dependencies is the ability to specify if it is required depending on the operating system or Python version in use. PEP 508 defines the scheme which has been implemented by this module.

Usage

>>> from packaging.markers import Marker, UndefinedEnvironmentName
>>> marker = Marker("python_version>'2'")
>>> marker
<Marker('python_version > "2"')>
>>> # We can evaluate a marker to see if the dependency is required
>>> marker.evaluate()
True
>>> # We can also override the environment
>>> env = {'python_version': '1.5.4'}
>>> marker.evaluate(environment=env)
False
>>> # Multiple markers can be ANDed
>>> and_marker = Marker("os_name=='a' and os_name=='b'")
>>> and_marker
<Marker('os_name == "a" and os_name == "b"')>
>>> # Multiple markers can be ORed
>>> or_marker = Marker("os_name=='a' or os_name=='b'")
>>> or_marker
<Marker('os_name == "a" or os_name == "b"')>
>>> # Markers can be also used with extras, to pull in dependencies if
>>> # a certain extra is being installed
>>> extra = Marker('extra == "bar"')
>>> # Evaluating an extra marker with no environment is an error
>>> try:
...     extra.evaluate()
... except UndefinedEnvironmentName:
...     pass
>>> extra_environment = {'extra': ''}
>>> extra.evaluate(environment=extra_environment)
False
>>> extra_environment['extra'] = 'bar'
>>> extra.evaluate(environment=extra_environment)
True

Reference

class packaging.markers.Marker(markers)

This class abstracts handling markers for dependencies of a project. It can be passed a single marker or multiple markers that are ANDed or ORed together. Each marker will be parsed according to PEP 508.

Parameters:markers (str) – The string representation of a marker or markers.
Raises:InvalidMarker – If the given markers are not parseable, then this exception will be raised.
evaluate(environment=None)

Evaluate the marker given the context of the current Python process.

Parameters:environment (dict) – A dictionary containing keys and values to override the detected environment.
Raises:UndefinedComparison: If the marker uses a PEP 440 comparison on strings which are not valid PEP 440 versions.
Raises:UndefinedEnvironmentName: If the marker accesses a value that isn’t present inside of the environment dictionary.
exception packaging.markers.InvalidMarker

Raised when attempting to create a Marker with a string that does not conform to PEP 508.

exception packaging.markers.UndefinedComparison

Raised when attempting to evaluate a Marker with a PEP 440 comparison operator against values that are not valid PEP 440 versions.

exception packaging.markers.UndefinedEnvironmentName

Raised when attempting to evaluate a Marker with a value that is missing from the evaluation environment.