I recently read a line of code and do not understand what is it meaning, the code is
import collections
d = collections.defaultdict(lambda : [-1, -1, -1])
I'm confused about the lambda here. I only know lamnda x: x+1. Usually, we have an argument for lambda. What if we don't have argument like this code?
Thanks a lot,
Lambda function can be used efficiently even without the lambda variable in python. General procedure for usage of lambda function is as given
def identity(x):
return x+1
lambda x:x+1
lambda :x
will return same as x.
Without a variable the lambda function stays idle without any action an hence it can be used without a variable for getting the same answer. In precise usage of lambda without a variable of action is just an extra line of code.But it is valid.
Yes, we can define a lambda function without any argument. But, it will be useless because there will be nothing to operate on. Let’s have a look at a simple example.
get_5 = lambda: 5
print(get_5()) # 5
Since the lambda function is always returning the same value, we can just assign it a variable. Using lambda function without any argument is plain abuse of this feature.
d = collections.defaultdict([-1, -1, -1])
produces the error:
TypeError: first argument must be callable or None
since list isin't callable, but lambda functions are callable.
[-1, -1, -1], each time it was returned it would be a reference to the same object. But almost certainly, you want a fresh copy for each time, hence the lambda which accomplishes that. (Presumably, this is why the argument must be a callable, to avoid this pitfall.).