Last updated on December 20, 2020

When testing Python programs, coverage.py is often used in measuring code coverage, and enforcing 100% code coverage is regarded as a good practice:

# .coveragerc
[coverage:report]
# Enforce 100% coverage test
fail_under = 100
show_missing = True

However, if there are some lines of code that are platform dependent (i.e., they are never executed on at least one platform), code coverage tests usually fail. For example, the following code snippet would always lead to a coverage that is less than 100% on a platform other than Windows:

if os.name != 'nt':
    # Do something if the OS is not Windows...

You can ask coverage.py to ignore this block by adding a comment # pragma: no cover, but then coverage.py would ignore it on all platforms, including all non-Windows platforms. If you use tox for testing, this issue can be resolved cleanly.

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Last updated on October 3, 2020

In Python, FileNotFoundError is an exception that is raised when a requested file does not exist. Many people assume that when their programs fail to open a file in read-only mode or delete a file, FileNotFoundError must be raised and they should only need to process that. For example, some people would write code similar to:

def process_file(path):
    import sys

    try:
        f = open(path, 'r')  # or os.remove(path)
    except FileNotFoundError as e:
        print(f"File {path} not found!", file=sys.stderr)
        return
    # process the file...

However, this code may actually trigger unexpected errors. The reason is that, the failure to open a file in read-only mode or delete a file is not necessarily caused by the non-existence of the file. Very often, it's for different reasons: insufficient permission, or the file is a directory. In this case, PermissionError or IsADirectoryError would be thrown instead of FileNotFoundError. So, in the example above, one would want to catch all of them:

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