python 3 map/lambda method with 2 inputs

Question

I have a dictionary like the following in python 3:

ss = {'a':'2', 'b','3'}

I want to convert all he values to int using map function, and I wrote something like this:

list(map(lambda key,val: int(val), ss.items())))

but the python complains:

TypeError: () missing 1 required positional argument: 'val'

My question is how can I write a lambda function with two inputs (E.g. key and val)

Solution

ss.items() will give an iterable, which gives tuples on every iteration. In your lambda function, you have defined it to accept two parameters, but the tuple will be treated as a single argument. So there is no value to be passed to the second parameter.

1. You can fix it like this

   print(list(map(lambda args: int(args[1]), ss.items())))
   # [3, 2]
   

2. If you are ignoring the keys anyway, simply use ss.values() like this

   print(list(map(int, ss.values())))
   # [3, 2]
   

3. Otherwise, as suggested by Ashwini Chaudhary, using itertools.starmap,

   from itertools import starmap
   print(list(starmap(lambda key, value: int(value), ss.items())))
   # [3, 2]
   

4. I would prefer the List comprehension way

   print([int(value) for value in ss.values()])
   # [3, 2]
   

In Python 2.x, you could have done that like this

print map(lambda (key, value): int(value), ss.items())

This feature is called Tuple parameter unpacking. But this is removed in Python 3.x. Read more about it in PEP-3113

Comments

[jonrsharpe]

You've written it with two parameters just fine, but the problem is it is only being called with a single argument, a tuple (key, val).

[BrenBarn]

If you don't want to do anything with the keys, why don't you just map on ss.values()?

[Ashwini Chaudhary]

You need to use itertools.starmap instead of plain map as it unpacks the arguments for us(but only if you're using both keys and values, otherwise it's unnecessary as BrenBarn pointed out). In Python 2 it was possible using simple tuple argument unpacking: lambda (key, val): int(val).

[jfs]

Have you considered dict comprehension: {k: int(v) for k, v in ss.items()} Or if you want to modify the dict inplace: for k, v in ss.items(): ss[k] = int(v)

[jfs]

If you need only values then: values = map(int, ss.values())

[Mohammadreza]

Thanks every one for their correct comments. especially Ashwini,For the difference between python 2 and 3.

[Jon Clements]

Any other ways you fancy mentioning? :p

[jfs]

Option zero is to call ss.values(): result = map(int, ss.values())

[Mohammadreza]

Thank you for the holistic response. also thanks Ashwini for starmaps. I didnt know that difference in python 2 and 3. which was my main problem.