Testing for features of the environment at runtime

A computation can require a certain package to be installed in the runtime environment. Abstractly such a package describes a Feature which can be tested for at runtime. It can be of various kinds, most prominently an Executable in the PATH, a PythonModule, or an additional package for some installed system such as a GapPackage.

AUTHORS:

• Julian Rüth (2016-04-07): Initial version

• Jeroen Demeyer (2018-02-12): Refactoring and clean up

EXAMPLES:

Some generic features are available for common cases. For example, to test for the existence of a binary, one can use an Executable feature:

sage: from sage.features import Executable
sage: Executable(name="sh", executable="sh").is_present()
FeatureTestResult('sh', True)

Here we test whether the grape GAP package is available:

sage: from sage.features.gap import GapPackage
sage: GapPackage("grape", spkg="gap_packages").is_present()  # optional - gap_packages
FeatureTestResult('gap_package_grape', True)

Note that a FeatureTestResult acts like a bool in most contexts:

sage: if Executable(name="sh", executable="sh").is_present(): "present."
'present.'

When one wants to raise an error if the feature is not available, one can use the require method:

sage: Executable(name="sh", executable="sh").require()

sage: Executable(name="random", executable="randomOochoz6x", spkg="random", url="http://rand.om").require() # optional - sage_spkg
Traceback (most recent call last):
...
FeatureNotPresentError: random is not available.
Executable 'randomOochoz6x' not found on PATH.
...try to run...sage -i random...
Further installation instructions might be available at http://rand.om.

As can be seen above, features try to produce helpful error messages.

class sage.features.CythonFeature(name, test_code, **kwds)

Bases: sage.features.Feature

A Feature which describes the ability to compile and import a particular piece of Cython code.

To test the presence of name, the cython compiler is run on test_code and the resulting module is imported.

EXAMPLES:

sage: from sage.features import CythonFeature
sage: fabs_test_code = '''
....: cdef extern from "<math.h>":
....:     double fabs(double x)
....:
....: assert fabs(-1) == 1
....: '''
sage: fabs = CythonFeature("fabs", test_code=fabs_test_code, spkg="gcc", url="https://gnu.org")
sage: fabs.is_present()
FeatureTestResult('fabs', True)

Test various failures:

sage: broken_code = '''this is not a valid Cython program!'''
sage: broken = CythonFeature("broken", test_code=broken_code)
sage: broken.is_present()
FeatureTestResult('broken', False)

sage: broken_code = '''cdef extern from "no_such_header_file": pass'''
sage: broken = CythonFeature("broken", test_code=broken_code)
sage: broken.is_present()
FeatureTestResult('broken', False)

sage: broken_code = '''import no_such_python_module'''
sage: broken = CythonFeature("broken", test_code=broken_code)
sage: broken.is_present()
FeatureTestResult('broken', False)

sage: broken_code = '''raise AssertionError("sorry!")'''
sage: broken = CythonFeature("broken", test_code=broken_code)
sage: broken.is_present()
FeatureTestResult('broken', False)

class sage.features.Executable(name, executable, **kwds)

Bases: sage.features.FileFeature

A feature describing an executable in the PATH.

In an installation of Sage with SAGE_LOCAL different from SAGE_VENV, the executable is searched first in SAGE_VENV/bin, then in SAGE_LOCAL/bin, then in PATH.

Note

Overwrite is_functional() if you also want to check whether the executable shows proper behaviour.

Calls to is_present() are cached. You might want to cache the Executable object to prevent unnecessary calls to the executable.

EXAMPLES:

sage: from sage.features import Executable
sage: Executable(name="sh", executable="sh").is_present()
FeatureTestResult('sh', True)
sage: Executable(name="does-not-exist", executable="does-not-exist-xxxxyxyyxyy").is_present()
FeatureTestResult('does-not-exist', False)

absolute_filename()

The absolute path of the executable as a string.

EXAMPLES:

sage: from sage.features import Executable
sage: Executable(name="sh", executable="sh").absolute_filename()
'/...bin/sh'

A FeatureNotPresentError is raised if the file cannot be found:

sage: Executable(name="does-not-exist", executable="does-not-exist-xxxxyxyyxyy").absolute_path()
Traceback (most recent call last):
...
sage.features.FeatureNotPresentError: does-not-exist is not available.
Executable 'does-not-exist-xxxxyxyyxyy' not found on PATH.

is_functional()

Return whether an executable in the path is functional.

This method is used internally and can be overridden in subclasses in order to implement a feature test. It should not be called directly. Use Feature.is_present() instead.

EXAMPLES:

The function returns True unless explicitly overwritten:

sage: from sage.features import Executable
sage: Executable(name="sh", executable="sh").is_functional()
FeatureTestResult('sh', True)

class sage.features.Feature(name, spkg=None, url=None, description=None)

Bases: sage.features.TrivialUniqueRepresentation

A feature of the runtime environment

INPUT:

• name – (string) name of the feature; this should be suitable as an optional tag for the Sage doctester, i.e., lowercase alphanumeric with underscores (_) allowed; features that correspond to Python modules/packages may use periods (.)

• spkg – (string) name of the SPKG providing the feature

• description – (string) optional; plain English description of the feature

• url – a URL for the upstream package providing the feature

Overwrite _is_present() to add feature checks.

EXAMPLES:

sage: from sage.features.gap import GapPackage
sage: GapPackage("grape", spkg="gap_packages")  # indirect doctest
Feature('gap_package_grape')

For efficiency, features are unique:

sage: GapPackage("grape") is GapPackage("grape")
True

is_present()

Return whether the feature is present.

OUTPUT:

A FeatureTestResult which can be used as a boolean and contains additional information about the feature test.

EXAMPLES:

sage: from sage.features.gap import GapPackage
sage: GapPackage("grape", spkg="gap_packages").is_present()  # optional - gap_packages
FeatureTestResult('gap_package_grape', True)
sage: GapPackage("NOT_A_PACKAGE", spkg="gap_packages").is_present()
FeatureTestResult('gap_package_NOT_A_PACKAGE', False)

The result is cached:

sage: from sage.features import Feature
sage: class TestFeature(Feature):
....:     def _is_present(self):
....:         print("checking presence")
....:         return True
sage: TestFeature("test").is_present()
checking presence
FeatureTestResult('test', True)
sage: TestFeature("test").is_present()
FeatureTestResult('test', True)
sage: TestFeature("other").is_present()
checking presence
FeatureTestResult('other', True)
sage: TestFeature("other").is_present()
FeatureTestResult('other', True)

require()

Raise a FeatureNotPresentError if the feature is not present.

EXAMPLES:

sage: from sage.features.gap import GapPackage
sage: GapPackage("ve1EeThu").require()
Traceback (most recent call last):
...
FeatureNotPresentError: gap_package_ve1EeThu is not available.
`TestPackageAvailability("ve1EeThu")` evaluated to `fail` in GAP.

resolution()

Return a suggestion on how to make is_present() pass if it did not pass.

OUTPUT:

A string.

EXAMPLES:

sage: from sage.features import Executable
sage: Executable(name="CSDP", spkg="csdp", executable="theta", url="https://github.com/dimpase/csdp").resolution()  # optional - sage_spkg
'...To install CSDP...you can try to run...sage -i csdp...Further installation instructions might be available at https://github.com/dimpase/csdp.'

exception sage.features.FeatureNotPresentError(feature, reason=None, resolution=None)

Bases: RuntimeError

A missing feature error.

EXAMPLES:

sage: from sage.features import Feature, FeatureTestResult
sage: class Missing(Feature):
....:     def _is_present(self):
....:         return False

sage: Missing(name="missing").require()
Traceback (most recent call last):
...
FeatureNotPresentError: missing is not available.

resolution

Initialize self. See help(type(self)) for accurate signature.

class sage.features.FeatureTestResult(feature, is_present, reason=None, resolution=None)

Bases: object

The result of a Feature.is_present() call.

Behaves like a boolean with some extra data which may explain why a feature is not present and how this may be resolved.

EXAMPLES:

sage: from sage.features.gap import GapPackage
sage: presence = GapPackage("NOT_A_PACKAGE").is_present(); presence  # indirect doctest
FeatureTestResult('gap_package_NOT_A_PACKAGE', False)
sage: bool(presence)
False

Explanatory messages might be available as reason and resolution:

sage: presence.reason
'`TestPackageAvailability("NOT_A_PACKAGE")` evaluated to `fail` in GAP.'
sage: bool(presence.resolution)
False

If a feature is not present, resolution defaults to feature.resolution() if this is defined. If you do not want to use this default you need explicitly set resolution to a string:

sage: from sage.features import FeatureTestResult
sage: package = GapPackage("NOT_A_PACKAGE", spkg="no_package")
sage: str(FeatureTestResult(package, True).resolution)  # optional - sage_spkg
'...To install gap_package_NOT_A_PACKAGE...you can try to run...sage -i no_package...'
sage: str(FeatureTestResult(package, False).resolution) # optional - sage_spkg
'...To install gap_package_NOT_A_PACKAGE...you can try to run...sage -i no_package...'
sage: FeatureTestResult(package, False, resolution="rtm").resolution
'rtm'

resolution

Initialize self. See help(type(self)) for accurate signature.

class sage.features.FileFeature(name, spkg=None, url=None, description=None)

Bases: sage.features.Feature

Base class for features that describe a file or directory in the file system.

A subclass should implement a method absolute_filename().

EXAMPLES:

Two direct concrete subclasses of FileFeature are defined:

sage: from sage.features import StaticFile, Executable, FileFeature
sage: issubclass(StaticFile, FileFeature)
True
sage: issubclass(Executable, FileFeature)
True

To work with the file described by the feature, use the method absolute_filename(). A FeatureNotPresentError is raised if the file cannot be found:

sage: Executable(name="does-not-exist", executable="does-not-exist-xxxxyxyyxyy").absolute_path()
Traceback (most recent call last):
...
sage.features.FeatureNotPresentError: does-not-exist is not available.
Executable 'does-not-exist-xxxxyxyyxyy' not found on PATH.

A FileFeature also provides the is_present() method to test for the presence of the file at run time. This is inherited from the base class Feature:

sage: Executable(name="sh", executable="sh").is_present()
FeatureTestResult('sh', True)

absolute_filename()

The absolute path of the file as a string.

Concrete subclasses must override this abstract method.

absolute_path()

Deprecated alias for absolute_filename().

Deprecated to make way for a method of this name returning a Path.

EXAMPLES:

sage: from sage.features import Executable
sage: Executable(name="sh", executable="sh").absolute_path()
doctest:warning...
DeprecationWarning: method absolute_path has been replaced by absolute_filename
See https://trac.sagemath.org/31292 for details.
'/...bin/sh'

class sage.features.PythonModule(name, **kwds)

Bases: sage.features.Feature

A Feature which describes whether a python module can be imported.

EXAMPLES:

Not all builds of python include the ssl module, so you could check whether it is available:

sage: from sage.features import PythonModule
sage: PythonModule("ssl").require()  # not tested - output depends on the python build

class sage.features.StaticFile(name, filename, search_path=None, **kwds)

Bases: sage.features.FileFeature

A Feature which describes the presence of a certain file such as a database.

EXAMPLES:

sage: from sage.features import StaticFile
sage: StaticFile(name="no_such_file", filename="KaT1aihu", search_path=("/",), spkg="some_spkg", url="http://rand.om").require()  # optional - sage_spkg
Traceback (most recent call last):
...
FeatureNotPresentError: no_such_file is not available.
'KaT1aihu' not found in any of ['/']...
To install no_such_file...you can try to run...sage -i some_spkg...
Further installation instructions might be available at http://rand.om.

absolute_filename()

The absolute path of the file as a string.

EXAMPLES:

sage: from sage.features import StaticFile
sage: from sage.misc.temporary_file import tmp_dir
sage: dir_with_file = tmp_dir()
sage: file_path = os.path.join(dir_with_file, "file.txt")
sage: open(file_path, 'a').close() # make sure the file exists
sage: search_path = ( '/foo/bar', dir_with_file ) # file is somewhere in the search path
sage: feature = StaticFile(name="file", filename="file.txt", search_path=search_path)
sage: feature.absolute_filename() == file_path
True

A FeatureNotPresentError is raised if the file cannot be found:

sage: from sage.features import StaticFile
sage: StaticFile(name="no_such_file", filename="KaT1aihu", search_path=(), spkg="some_spkg", url="http://rand.om").absolute_filename()  # optional - sage_spkg
Traceback (most recent call last):
...
FeatureNotPresentError: no_such_file is not available.
'KaT1aihu' not found in any of []...
To install no_such_file...you can try to run...sage -i some_spkg...
Further installation instructions might be available at http://rand.om.

class sage.features.TrivialClasscallMetaClass

Bases: type

A trivial version of sage.misc.classcall_metaclass.ClasscallMetaclass without Cython dependencies.

class sage.features.TrivialUniqueRepresentation

Bases: object

A trivial version of UniqueRepresentation without Cython dependencies.

sage.features.package_systems()

Return a list of PackageSystem objects representing the available package systems.

The list is ordered by decreasing preference.

EXAMPLES:

sage: from sage.features import package_systems
sage: package_systems()    # random
[Feature('homebrew'), Feature('sage_spkg'), Feature('pip')]