Every now and then I see some code in a Python code base that uses assert statements to verify preconditions. This is dangerous for a few reasons. The main one being that if the Python interpreter is run with certain flags assert statements are not executed. For this reason calling functions in python asserts can be dangerous as shown by this example:

count = 0

def counter():
    global count
    print("counter function called")
    count += 1
    return count

counter()
print(count)
assert counter() is not None
print(count)

$ python assertions_experiment.py 
counter function called
1
counter function called
2

Running with the -O optimize command line option:

$ python -O assertions_experiment.py 
counter function called
1
1

It's worth noting that in all versions of Python assert is a statement and not a function. This means you can't easily change this behaviour later like you can for example with print functions. By all means keep using assert statements in your tests and perhaps in your prototypes but proceed with extreme caution in production code.