The local variable name hides the variable defined in the outer scope, making it inaccessible and might confuse.

Bad practice

filename = 'myfile.txt'

def read_file(filename):  # This shadows the global `filename`
    with open(filename) as file:
        return file.readlines()

Preferred:

FILENAME = 'myfile.txt'  # renamed global to UPPER_CASE as convention

def read_file(filename):
    with open(filename) as file:
        return file.readlines()

Bad practice

Another usual suspect of this is when you use the same parameter name inside a function as the global variable you are using. For example:

def run_app(app):
    # This `app` shadows the global app...
    app.run()

if __name__ == '__main__':
    app = MyApp()  # This is a global variable!
    run_app(app)

Preferred:

To avoid this re-defining of a global, consider not defining app as a global, but inside a main() function instead:

def run_app(app):
    # There is no longer a global `app` variable.
    app.run()

def main():
    app = MyApp()
    run_app(app)

if __name__ == '__main__':
    main()

Loco-CTO / Rystal-V6-Experiments

No ignore rules found