Python - Wrong number of arguments exception?

Question

So I have a function like:

def my_code(arg1, *args):
    ....

And I want this function to only be able to take either 2 or 3 arguments (that means *args can only be 1 or 2 arguments). How do I throw an error message if the number of arguments is wrong? If I use a try/exception, is there a specific exception type for this?

If you want to only take 2 or 3 arguments, it sounds like it may be better to define the function as `def func(arg1, arg2, arg3=None)`, or to write two functions. — user2357112, Mar 15 '18 at 21:25
The way to figure this out yourself is to write a function that takes two parameters, call it with one argument or four, and see what exception gets raised. You want to raise the same exception, with similar text (but maybe more informative—the exact text doesn't matter. just the exception type). — abarnert, Mar 15 '18 at 21:26

Olivier Melançon · Accepted Answer · 2018-03-15T22:20:55.870

2

You can get the length of args with len as you would for any tuple.

def my_code(arg1, *args):
    if not 0 < len(args) < 3:
        raise TypeError('my_code() takes either 2 or 3 arguments ({} given)'
                        .format(len(args) + 1))

my_code(1) # TypeError: my_code() takes either 2 or 3 arguments (1 given)
my_code(1, 2) # pass
my_code(1, 2, 3) # pass
my_code(1, 2, 3, 4) # TypeError: my_code() takes either 2 or 3 arguments (4 given)

edited Mar 15 '18 at 22:20

answered Mar 15 '18 at 21:27

Olivier Melançon

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To bring the error inline with the builtin argument checking, I would suggest `raise TypeError('my_code() takes either 2 or 3 arguments (%d given)' % (len(args) + 1))` – TemporalWolf Mar 15 '18 at 22:19
If you really want to get fancy, you can `import sys` and use [`sys._getframe().f_code.co_name`](https://stackoverflow.com/a/13514318/3579910) to programmatically pull the function name... although that's probably overkill depending on the size of the project. – TemporalWolf Mar 15 '18 at 22:43
I think you can actually do simpler with inspect: `inspect.stack()[0][3]` – Olivier Melançon Mar 15 '18 at 22:46

score 0 · Answer 2 · answered Mar 15 '18 at 21:27

Your test is:

if len(args) not in (1,2):

though there are of course other ways to phrase that.

As for the exception, if you call a built-in function with the wrong number of arguments, you get a TypeError. If your application doesn't justify creating your own subclass of Exception, then that is probably the way to go.

score 0 · Answer 3 · answered Mar 15 '18 at 21:29

0

def my_code(*args): if len(args)>2: raise TypeError else: #code for your function pass

Basically *args is a tuple, and if you want a maximum number of arguments you can raise a TypeError

answered Mar 15 '18 at 21:29

dejanualex

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You don't need the `else`. – Klaus D. Mar 15 '18 at 21:36

Python - Wrong number of arguments exception?

3 Answers3