Pitfalls

Don’t define data structures in function parameters. Only constants.

def foo(x=[]):
    x.append('test')
    print(x)

foo()
# => ['test']
foo()
# => ['test', 'test']

Best practice way:

def foo(x=None):
    if x is None:
        x = []
    x.append('test')
    print(x)

foo()
# => ['test']
foo()
# => ['test']