lambda in python can iterate dict?

Question

I have an interview recently. The interviewer asked me the ways to iterate dict in python. I said all the ways use for statement. But he told me that how about lambda?

I feel confused very much and I consider lambda as an anonymity function, but how it iterates a dict? some code like this:

new_dict = sorted(old_dict.items(), lambda x: x[1]) # sorted by value in dict

But in this code, the lambda is used as a function to provide the compared key. What do you think this question?

Answer 1

You don't iterate with lambda. There are following ways to iterate an iterable object in Python:

1. for statement (your answer)
2. Comprehension, including list [x for x in y], dictionary {key: value for key, value in x} and set {x for x in y}
3. Generator expression: (x for x in y)
4. Pass to function that will iterate it (map, all, itertools module)
5. Manually call next function until StopIteration happens.

Note: 3 will not iterate it unless you iterate over that generator later. In case of 4 it depends on function.

For iterating specific collections like dict or list there can be more techniques like while col: remove element or with index slicing tricks.

Now lambda comes into the picture. You can use lambdas in some of those functions, for example: map(lambda x: x*2, [1, 2, 3]). But lambda here has nothing to do with iteration process itself, you can pass a regular function map(func, [1, 2, 3]).