Hey there, let's break down why you're seeing this frustrating mismatch—you've installed matplotlib, but Python can't find it. Let's walk through the steps to fix this:

First, Understand the Discrepancy #

Looking at your command line output:

C:\Users\Username>py
Python 3.8.0 (tags/v3.8.0:fa919fd, Oct 14 2019, 19:21:23) [MSC v.1916 32 bit (Intel)] on win32
>>> import matplotlib
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'matplotlib'

C:\Users\Username>pip install matplotlib
Requirement already satisfied: matplotlib in c:\program files (x86)\python38-32\lib\site-packages (3.2.0rc1)

You're running a 32-bit Python 3.8, and pip confirms matplotlib is installed in that same Python's site-packages folder. The issue is likely either a mismatch between your pip and py interpreter, a corrupted installation, or a path configuration problem.

Step 1: Align pip with Your Python Interpreter #

The most common culprit is multiple Python versions on your system, so the pip you're using isn't linked to the py interpreter you're running. Fix this by using the pip associated directly with your py instance:

• Run this command to force a clean reinstall:

  py -m pip install matplotlib --force-reinstall
  

  The --force-reinstall flag overwrites any potentially corrupted files, ensuring a complete, working installation.

Step 2: Verify Python's Module Search Path #

If the above doesn't work, check if Python is actually looking in the right directory for modules:

1. Launch Python with py
2. Run these commands in the interactive shell:

   import sys
   print(sys.path)
   

3. Look for c:\program files (x86)\python38-32\lib\site-packages in the output. If it's missing:
   • Temporarily add it to test if this fixes the issue:

     sys.path.append("c:\\program files (x86)\\python38-32\\lib\\site-packages")
     import matplotlib
     

   • If this works, you'll need to fix your Python's path permanently (check your system environment variables or create a .pth file in your Python's site-packages directory to add the path, though this is rare for standard installations).

Step 3: Check for Permission Issues #

Sometimes installing without admin rights can lead to incomplete file writes. Try this:

1. Open Command Prompt as Administrator
2. Run the reinstall command again:

   py -m pip install matplotlib --force-reinstall
   

Step 4: Confirm Python and Pip Versions #

Double-check that py and pip are pointing to the same Python version:

• Run py --version (should show 3.8.0 32-bit)
• Run pip --version (should show it's associated with Python 3.8.0 in c:\program files (x86)\python38-32)
• If they don't match, stick to using py -m pip exclusively to avoid future mismatches.

内容的提问来源于stack exchange，提问作者Sudheer