Why I cannot import a library that is in my pip list?

Question

I have just installed pypng library with sudo -E pip install pypng . I see this library in the list when I execute pip list (the version that I see there is 0.0.18 ).

When I start python (or ipython) session and execute

import pypng

I get

ImportError: No module named pypng

Answer 1

According to docs on github you need to import png.

import png
png.from_array([[255, 0, 0, 255],
                [0, 255, 255, 0]], 'L').save("small_smiley.png")

Answer 2

I think the module name is just png. Try the following:

import png

Answer 3

Library name is not always the name of installed package. For the pypng it is just png , as noted in the documentation .

In case, if the documentation does not provide enough info, you could use pip show in order to list all installed files for specified package:

$ python -m pip show -f pypng                                                  
Metadata-Version: 2.0
Name: pypng
Version: 0.0.18
Summary: Pure Python PNG image encoder/decoder
Home-page: https://github.com/drj11/pypng
Author: David Jones
Author-email: drj@pobox.com
License: UNKNOWN
Location: /usr/lib/python3.5/site-packages
Requires: 
Files:
  __pycache__/png.cpython-35.pyc
  png.py
  pypng-0.0.18.dist-info/DESCRIPTION.rst
  pypng-0.0.18.dist-info/INSTALLER
  pypng-0.0.18.dist-info/METADATA
  pypng-0.0.18.dist-info/RECORD
  pypng-0.0.18.dist-info/WHEEL
  pypng-0.0.18.dist-info/metadata.json
  pypng-0.0.18.dist-info/top_level.txt

As you can see, there is png.py file, which is being imported with import png .