Python underscore as a function parameter

Question

I have a python specific question. What does a single underscore _ as a parameter means? I have a function calling hexdump(_). The _ was never defined, so I guess it has some special value, I could not find a reference telling me what it means on the net. I would be happy if you could tell me.

Answer 1

From what I've been able to figure out, it seems like this is the case:

_ is used to indicate that the input variable is a throwaway variable/parameter and thus might be required or expected, but will not be used in the code following it.

For example:

# Ignore a value of specific location/index  for _ in range(10)      print("Test")    # Ignore a value when unpacking  a,b,_,_ = my_method(var1)  

(Credit to this post)

The specific example I came across was this:

    def f(_):         x = random() * 2 - 1         y = random() * 2 - 1         return 1 if x ** 2 + y ** 2 < 1 else 0 

Answer 2

In Python shells, the underscore (_) means the result of the last evaluated expression in the shell:

>>> 2+3
5
>>> _
5

There's also _2, _3 and so on in IPython but not in the original Python interpreter. It has no special meaning in Python source code as far as I know, so I guess it is defined somewhere in your code if it runs without errors.