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Importing value of mutable attribute

ID: py/import-of-mutable-attribute
Kind: problem
Security severity: 
Severity: warning
Precision: medium
Tags:
   - reliability
   - maintainability
   - modularity
Query suites:
   - python-security-and-quality.qls

Click to see the query in the CodeQL repository

Explicitly importing an attribute from a module into the current namespace means that the value of that attribute will not be updated if the value in the original module changes.

This can mean that changes in global state are not observed locally, which may lead to inconsistencies and possible errors.

Recommendation

Instead of using from module import attr, simply import the module using import module and replace all uses of attr with module.attr.

Example

In the first of the two modules shown below, from sys import stdout is used to import the stdout attribute, rather than using import sys to import the module. Then stdout is used in the main() function.

from sys import stdout

def main():
    stdout.write("Hello World!")

In the second module, below, a function, redirect_to_file is defined to collect the output from sys.stdout and save it to a file. However, redirect_to_file will not work correctly when passed the main() function. This is because the main() function will not see the change to sys.stdout, as it uses its own version of stdout that was defined when the module was loaded.

import sys

def redirect_to_file(function, args, kwargs, filename):
    with open(filename) as out:
        orig_stdout = sys.stdout
        sys.stdout = out
        try:
            function(*args, **kwargs)
        finally:
            sys.stdout = orig_stdout

The problem can be fixed by rewriting the first module to import the sys module and write to sys.stdout, as shown below.

import sys

def main():
    sys.stdout.write("Hello World!")

References

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