Why Does `os.listdir` Cause `FileNotFoundError` When Opening Files in Python?

Understanding FileNotFoundError Exception When Using os.listdir and open

In Python, while iterating over files in a directory using os.listdir, users may encounter a FileNotFoundError. This is because os.listdir only returns the filename, not the full path.

Consider the following code:

import os

path = r'E:/somedir'

for filename in os.listdir(path):
    f = open(filename, 'r')

When running this code, Python will raise a FileNotFoundError, even though the file exists. This is because open expects the full path to the file, which is 'E:/somedir/foo.txt' when dealing with the file 'foo.txt'.

To resolve this issue, use os.path.join to prepend the directory to the filename:

path = r'E:/somedir'

for filename in os.listdir(path):
    with open(os.path.join(path, filename)) as f:
        # process the file

Additionally, the code can automatically close the file using a with block.

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