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.

Solution 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

Solution 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.

Solution 3

underscore is considered a 'don't care' variable, furthermore IDEs like PyCharm will not give a warning for it if it is unused

so in a function

def q(a, b, _, c):
    pass

the IDE will underline a,b and c (unused parameter) but not the underscore

why would you use it and not omit the parameter?

->when you inherit from some class and want to override a function where you don't want to use some parameter

other common use is to indicate you don't want to use a part of a tuple when you iterate (or other unpacking) - this reduces clutter

names_and_food = [('michael', 'fruit'), ('eva', 'vegetables')]
for name, _ in names_and_food:
    print(name)

I cant find it in any python PEP, but pylint has it even in the FAQ

Solution 4

It doesn't have a special value in the code you write. It stores the result of the last expression you evaluated in your interactive interpreter and is used for convenience

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Comments

[Marco]

To people answering this: what does a single underscore as a parameter mean in ("normal") python code, and not when used in the interpreter?

[Noufal Ibrahim]

The interpreter you get when you type python at the prompt and hit enter. Try typing 3+4 enter at the >>> prompt and hitting enter. Then trying printing the value of _. You'll get 7 (result of last expression evaluated).

[Hagbard]

Why do you think this is not answering the question? In my opinion, it does.

[Derte Trdelnik]

What does a single underscore _ as a parameter means? - underscore as a function parameter, not as a statement (in interactive interpreter)

[chepner]

@DerteTrdelnik It's still a variable, even if it's the entire expression serving as an expression statement. (Also, the original question asks about an argument to a function, despite using the word parameter.)

[pcko1]

amazing how I came here for the exact same example (found in github.com/apache/spark/blob/master/examples/src/main/python‌​/…) haha