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

FileNotFoundError when Iterating Over Files with os.listdir

When iterating over files in a directory using os.listdir, it's possible to encounter a FileNotFoundError even though the file exists. This is because os.listdir only returns the file name, not the full path to the file.

import os

path = r'E:/somedir'

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

In this example, Python throws a FileNotFoundError even though the file exists because filename only contains the file name, such as 'foo.txt', instead of the full path, such as 'E:/somedir/foo.txt'.

To resolve this issue, use os.path.join to prepend the directory path 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, it's recommended to use a context manager with statement for opening files, as it ensures the file is closed properly when finished.

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