Unlocking Python’s Underscore: 4 Powerful Uses You Should Know

1. Represent the value of the last expression

In Python the underscore (_) holds the result of the most recent expression that was not assigned to a variable, similar to the $? variable in a Linux shell.

>> 3+5
8
>>> _
8
>>> _*3
24
>>> _
24

Note that the underscore only captures results of expressions without an explicit variable; if a variable receives the result, the underscore still reflects the previous value.

>> x = 3+8
>>> _
24

2. Discard unwanted values

The underscore can act as a “black hole” to ignore values you do not need.

x, _, y = (1, 2, 3)

Only x and y are kept.

for _ in range(100):
    ...

In a for loop the underscore indicates that the loop variable is unused.

3. Indicate private functions and variables

When writing modules, a leading underscore marks names that should not be exported with from module import *. Double underscores trigger name‑mangling.

_var1 = 100
var2 = 200

def func1():
    print(_var1)
    print(var2)

def __func2():
    print(_var1)
    print(_var2)

>> from moda import *
>>> var2
200
>>> _var1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name '_var1' is not defined
>>> func1()
100
200
>>> __func2()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name '__func2' is not defined

4. Separate large numeric literals

Since Python 3.6 you can use underscores inside numeric literals to improve readability, similar to commas in other languages. They work for decimal, binary, octal, and hexadecimal literals.

a = 1_000_000
print(a)          # 1000000

b = 0b_111_1110
print(b)          # 126
print(bin(b))     # 0b1111110

o = 0o12_34
print(o)          # 668
print(oct(o))     # 0o1234

h = 0x_0a_ef
print(h)          # 2799
print(hex(h))     # 0xaef

These underscore usages make large numbers easier to read.