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A library to call Python functions with parameters determined at runtime by name.

Project description


calllib provides 3 functions, apply, getargs, and inspect_params. These functions are used to inspect and call Python functions whose parameters are not known in advance, but are determined at runtime from a mapping of available objects.

This is particularly useful in plug-in frameworks, where you don’t want every callback function to be required to have identical signatures. Instead, each function can take a subset of available parameters.

For example:

>>> from __future__ import print_function
>>> import calllib

>>> def callback1(time):
...    print('callback1 called at:', time)

>>> def callback2(time, reason):
...    print('callback2 called at:', time, 'reason:', reason)

# register the callbacks
>>> callbacks = [callback1, callback2]

# elsewhere: compute the total universe of possible
#  callback arguments
>>> args = {'time': 'noon', 'reason': 'abort'}
>>> for callback in callbacks:
...    calllib.apply(callback, args)  # execute each callback
callback1 called at: noon
callback2 called at: noon reason: abort

The last line shows that you can call any callback routine without knowing its exact arguments, as long as its arguments are a subset of the available arguments.


apply(callable, args)

  • callable - Any callable object that can be inspected with the inspect module.
  • args - A mapping object, typically a dict, that contains the available arguments that will be passed to callable.

callable is inpsected with getargs and the its parameters are extracted by name. For each parameter the corresponding value is retrieved from args by name and passed to the callable.

apply returns the result of executing callable with the computed arguments.


getargs(callable, args)

  • callable - Any callable object that can be inspected with the inspect module.
  • args - A mapping object, typically a dict, that contains the available arguments that could be passed to callable.

callable is inspected to determine its parameters. For each parameter the corresponding value is retrieved from args. If a parameter is not found in args callable has a default value for that parameter, the default value is retrieved.

getargs returns a list of actual argument values that would be passed to callable.



  • callable - Any callable object that can be inspected with the inspect module.

callable is inspected to deterine its parameters and default values, if any. inspect_params returns a tuple containing a list of parameter names and a dict with default values, if any. For example:

>>> def foo(x, y=0, z=6): pass
>>> calllib.inspect_params(foo)
(['x', 'y', 'z'], {'y': 0, 'z': 6})

>>> class Baz(object):
...     def __init__(self, x, y='hello'): pass
>>> calllib.inspect_params(Baz)
(['x', 'y'], {'y': 'hello'})

Types of callables supported

calllib supports any callable written in Python. This includes functions, bound and unbound methods, classes, and object instances with __call__ members.

Because they are not introspectable by the inspect module, built in Python functions such as len cannot be used with apply.

Default arguments

Functions with default arguments are fully supported by calllib. If an argument is not specified in the args parameter to apply and it has a default, the default value will be used.


To test, run ‘python test’. On python >= 3.0, this also runs the doctests.

Change log

1.8 2016-10-27 Eric V. Smith

  • Remove hack for changing RPM name (issue #7).
  • Always require setuptools (issue #6).
  • No code changes.

1.7 2015-05-16 Eric V. Smith

  • Removed ‘test’ package, so it won’t get installed by bdist_*. It’s still included in sdists.
  • No code changes.

1.6 2015-05-15 Eric V. Smith

  • Changed RPM name to python3-calllib if running with python 3.
  • No code changes.

1.5 2014-12-07 Eric V. Smith

  • Added inspect_params (issue #5).

1.4 2014-07-24 Eric V. Smith

  • Release version 1.4. No code changes.
  • Add a README.txt entry on running the test suite.
  • Fix missing test/ in the sdist.

1.3 2014-03-14 Eric V. Smith

  • Add to, so it will be included in sdists (issue #4).

1.2 2014-02-12 Eric V. Smith

  • New release just to update development status classifier.

1.1 2014-02-12 Eric V. Smith

  • Produce an RPM named python-calllib (issue #3).
  • Support python3 (issue #2).
  • Moved tests to a separate module (issue #1).

1.0 2011-11-10 Eric V. Smith

  • Finalized API.
  • Added tests for derived classes.

0.2 2011-11-10 Eric V. Smith

  • Allow for classes with no __init__ method.
  • Normalize test names.

0.1 2011-11-09 Eric V. Smith

  • Initial release.

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