Passing a dictionary to a function with unpacking argument

Question

This below snippet of code gives me this error TypeError: pop() argument after ** must be a mapping, not tuple.

class a():
    data={'a':'aaa','b':'bbb','c':'ccc'}
    def pop(self, key, **args):
        return self.data.pop(key, **args)

b=a()
print(b.pop('a',{'b':'bbb'}))

But when I replace double ** with single *, this works fine. As per my understanding , if we are passing a dictionary , we should have double **. In this case the second argument what's being passed is dictionary {'b':'bbb'}. Then how is it throwing error in first case but not in second case?

class a():
    data={'a':'aaa','b':'bbb','c':'ccc'}
    def pop(self, key, *args):
        return self.data.pop(key, *args)

b=a()
print(b.pop('a',{'b':'bbb'})

With your first example, I get `TypeError: pop() takes 2 positional arguments but 3 were given`. This is because `{'b':'bbb'}` is passed as a _positional_ argument, not a keyword argument. In `**args`, the `**` indicates that you are taking keyword arguments. These keyword arguments are available in the function as a dictionary. E.g., `pop('a', b='bbb')` would give you `args = {'b': 'bbb'}` inside `pop()`. It has nothing to do with passing dictionaries. — Pranav Hosangadi, Jul 27 '21 at 20:06
When you use `**args` in the `a.pop()` definition, it means that the function takes keyword arguments after `key`. But you aren't calling it with any keywords. And what are you specting `**args` to mean when you pass it to `self.data.pop()`? — Barmar, Jul 27 '21 at 20:11
"As per my understanding , if we are passing a dictionary , we should have double `**`" Your understanding is wrong. Double-asterisks indicate *variadic keyword arguments*, i.e., any number of keyword arguments. so if you do `def foo(**args): ..` you can call `foo` in the following ways: `foo(); foo(a=1); foo(banana=[], bar=2)` and so on. These arguments will be provided **as a dictionary**. Now, you can *unpack* mappings (e.g. a dict) using `**a_mapping`, as an argument to a function. They will be *passed as keyword arguments*, and handled in different ways depending on the exact signature — juanpa.arrivillaga, Jul 27 '21 at 20:29
@juanpa.arrivillaga Then should not this work ? ``` class a(): data={'a':'aaa','b':'bbb','c':'ccc'} def pop(self, key, *args): return self.data.pop(key, *args) b=a() print(b.pop('a',1,2,3)) ``` — Kumar, Jul 27 '21 at 20:41
@Kumar no, it shouldn't. Have you *read the signature for `dict.pop`*? `dict.pop` accepts a key to pop, and a default value to return if the key doesn't exist. So at most, two positional arguments. When you do `b.pop('a', 1, 2, 3)` you are passing *4* positional arguments. So it will raise a TypeError. You seem to not understand the `dict.pop` method, it's hard to say what you are *expecting*. — juanpa.arrivillaga, Jul 27 '21 at 20:56

score 2 · Answer 1 · answered Jul 27 '21 at 20:07

2

If you want a dictionary to be used as keyword arguments, you have to use the ** in the call as well:

print(b.pop('a',**{'b':'bbb'}))

But I don't think that's really what you wanted anyway.

answered Jul 27 '21 at 20:07

Tim Roberts

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Passing a dictionary to a function with unpacking argument

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