Recursive calls and variable assignment errors [Python]

Question

I was just playing around testing if strings were immutable because I was a bit tired :D and made this

def x(string = "a", y = 0):
    if y == 5:
        return
    else:
        string += "x"
        print(string)
        x(string, y = y + 1)
        print(string)
x()

and I am simply wondering why this works but

def x(string = "a", y = 0):
    if y == 5:
        return
    else:
        string += "x"
        print(string)
        x(string, y += 1)
        print(string)
x()

Does not work (The difference in the y variable assignment in the recursive call). Why is it a syntax error, theyre simply doing the same thing?

syntax error means python cannot accept this input in its grammar. — gzc
– gzc, Commented Feb 19, 2017 at 8:50
That's a lot of code to produce a syntax error. Try to produce the same error with as little code as possible, forget about recursion and strings. — Open AI - Opting Out
– Open AI - Opting Out, Commented Feb 19, 2017 at 9:15
Try printing y after the recursive call to x() and you will find it hasn't changed. You are using the keyword arg y not assignment to y in your x() function call. — AChampion
– AChampion, Commented Feb 19, 2017 at 9:34

Carles Mitjans · Accepted Answer · 2017-02-19 10:35:20Z

2

In the first version, you tell the function that your parameter y takes the value of your variable y, +1. If your parameter was called z, you'd have x(string, z=y+1).

As you see, y+=1 doesn't work, there you're attempting to modify your variable, not to give the function a named parameter. But += doesn't return anything, so the syntax is indeed incorrect.

edited Feb 19, 2017 at 10:35

Carles Mitjans

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answered Feb 19, 2017 at 8:51

Cedric Druck

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2 Comments

user7404408 Over a year ago

but y = y + 1 doesn't return anything either

Cedric Druck Over a year ago

y in this case is a parameter name, like my_param=y+1, it's not an assignment but passing a named parameter to a function ..

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