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Python Function - Dictionary as Input - Return Error - Interesting Behavior

Good day, brilliant Stack Overflow Community. I noticed an interesting behavior regarding Python function when input as a dictionary.

When we use the empty list as a default argument, we can return that list while appending some value.

def func_list(value,input=[]):
    return input.append(value)

I would expect the same thing to apply when empty dictionary as an argument. such as:

def func_dict(value,input={}):
    return input[value] = value

However, Python will raise a Syntax Error, and I can only do something like:

def func_dict(value,input={}):
    input[value] = value
    return input

I am wondering why? Thank you so much!

PS. Please feel free to modify the question, and apologize if my expression of the question is not clear enough!

Solution

  • The key difference is that input.append(value) is an expression and input[value] = value is not (it is just a statement). The return statement can only take an expression as a parameter (or no parameters to return None).

    You CAN do this:

    def func_dict(value,input={}):
    return input.update({ value: value }) or input

    How this works is that dict.update() returns None so we return the (now updated) input instead. I know it's ugly but can't think of a better one.