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 the marker to see if it is satisfied
>>> 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)[source]¶ 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 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
[source]¶ Raised when attempting to create a
Marker
with a string that does not conform to PEP 508.