Skip to content


Switch branches/tags

Latest commit


Git stats


Failed to load latest commit information.
Latest commit message
Commit time

Pydoc Checker

A Python module validating the arguments of methods and functions.

What is it?

Pydoc Checker modifies the functions and methods in your code adding a run-time validation of the type of the arguments, of the return value, and of the exceptions raised. The required types must be described in the pydoc of the method.

What should I do?

You should record the expected types in the pydoc of the function or method, like in this example.

def retrieve(hash_table, key, default_value=None):
    """Retrieve the element associated to key from hash_table.

    hash_table (HashTable): the hash table in which to search.
    key (unicode): the key to search.
    default_value (HashTableValue|None): if key is not in hash_table,
        and default_value is not None, default_value is returned.

    return (HashTableValue|None): the object associated to key.

    raise (KeyError): if key is not in hash_table and no default_value
        is provided.


If Pydoc Checker was called after the definition of this function, then right before each call to retrieve, some code will be executed to ensure that all passed values are of the correct type. Also, after retrieve was executed, Pydoc Checker will ensure that the return value is of the correct type, or, if the function raised, that the exception was expected.

In addition to this, it also checks the default argument values, like None for default_value in the example.

To enable Pydoc Checker, you just need to call check_all(pkgs) at the beginning of your program, where pkgs is a list of the (top-level) packages you want to check. Subpackages will be checked too.

#!/usr/bin/env python

"""Entry point of the program."""

from myprogram import run

if __name__ == "__main__":
    import pydocchecker

Finally, you should use only new-style classes (that is, a class should derive, directly or indirectly, from object.

It sounds like a lot of work

Nobody said it was easy to write maintainable code that can be understand by a new member of a team. Having proper pydoc strings allows to have this nice check, but, if you do not do it already, the real advantage is giving you incentives to write proper code documentation.

If I wanted a typed language, I would have used Java

If you pass the wrong object to a function, it is wrong both in Java and in Python.

Python is awesome for fast, short scripts. It is great for small project. It starts to have some drawbacks for medium to large project. Having additional confidence on such a Python project is never a wrong idea.

Will it slow down my program?

Depending on your style of programming, it could add a noticeable overhead. I suggest to run Pydoc Checker during manual and automated testing.

How do I write a type annotation?

The recognizable type annotations respect the following grammar.

<type> := [a-zA-Z0-9_]+ |
          <list> |
          <tuple> |
          <set> |
          <dict> |
<list> := [<type>]
<tuple> := (<type>, <type>, ..., <type>)
<set> := (<type>)
<dict> := {<type>: <type>} |
          {<id>: <type>, ..., <id>: <type>}

There are two kinds of dictionaries: if a single pair (key, value) is given, then it is assumed to be an "homogeneous" dict, that is, all keys are of the same type and all values are of the same type; if there are zero, two, or more pairs, then it is assumed to be a string-indexes associative array, and the type is satisfied if all keys are present in the actual dictionary, and the type of the values match.

Where do I see my errors?

Errors (that is, passing or returning the wrong type, or an unexpected exception) are outputted using the warn function of the warnings module.

If you choose to see debug information on the internals of Pydoc Checker, then the logs are printed using q, so by default are streamed to /tmp/q.

Is it configurable?

check_all accepts these optional arguments.

  • none_always_valid (boolean, default False): whether to consider None an instance of any type, instead then just of object and NoneType. In the latter case, use an annotation like <type>|None to accept also None values.

  • complain_for_missing_pydoc (boolean, default False): whether to log a warning also for missing pydocs, or missing type descriptions.

  • debug (integer, default 0): the higher, the more log lines will be printed in /tmp/q; valid values are between 0 and 5.

How do I install it?

To install Pydoc Checker, just clone the repository or download a tarball, and run the following.

sudo ./ install


  • Fallback on the pydoc of the parent class' method if the child does not have a pydoc or does not describe some argument.

  • Unambiguous syntax for the two types of dictionaries.

  • Better checking when two classes have the same name. At the moment, passing an object of any of the two types is accepted, but there should be some heuristic using the imports of the specific module.

  • Check also inner functions.

  • Check for exceptions, making sure that a warning is issued only if at least one type annotation is found in the pydoc.

  • If a type is not found, and there is not a default argument for that type, postpone the error to when the function is called, as it might just be that the type is not imported (and the function is never called).


A Python module validating the arguments of methods and functions.







No releases published


No packages published