How can I list all packages/modules available to Python from within a Python script?

Solution 1:

Alright, I was curious, and I digged a bit into pkgutil, and I came up with this, which is much simpler than I expected:

list(pkgutil.iter_modules())

It lists all top-level packages/modules available either as regular files or zip packages, without loading them. It will not see other types of packages though, unless they properly register with the pkgutil internals.

Each returned entry is a 3-tuple with the items:

1. module_finder: The file finder instance that found the module
2. name: The name of the module
3. ispkg: A boolean specifying whether it is a regular module or a package.

Example 3-tuple:

(FileFinder('/usr/lib/python3/dist-packages'), 'PIL', True)

And I can confirm that this did not load the PIL package:

In [11]: sys.modules['PIL']
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-11-b0fc0af6cc34> in <module>()
----> 1 sys.modules['PIL']

KeyError: 'PIL'

Solution 2:

I put together a very rough way of getting this list (see below), which appears to be more accurate than pkgutil. See details below.

In addition, I found loaded_modules and list-imports, but I tested none of them.

I have compared the results of my method with the answer by spectras:

1. All items in the output by spectras (say, modlist2) are in the output here (say, modlist1).
2. There are quite a few items in modlist1 that are not in modlist2. To my surprise, this difference included modules like sys, math, zlib, etc. In my case, the respective lengths were 390 vs. 327, so the method with pkgutil gives quite incomplete results.

The method to pull the list of available modules consists of:

1. Capturing output of help into a string
2. Removing spare text from the captured string
3. Splitting multicolumn output

Code is here:

def modules_list() :
    """Return a list of available modules"""
    import sys
    # Capture output of help into a string
    import io
    stdout_sys = sys.stdout
    stdout_capture = io.StringIO()
    sys.stdout = stdout_capture
    help('modules')
    sys.stdout = stdout_sys
    help_out = stdout_capture.getvalue()
    # Remove extra text from string
    help_out = help_out.replace('.', '')
    help_out = help_out.replace('available modules', '%').replace('Enter any module', '%').split('%')[-2]
    # Split multicolumn output
    help_out = help_out.replace('\n', '%').replace(' ', '%').split('%')
    help_out = list(filter(None, help_out))
    help_out.sort()
    return help_out