What is the purpose of the single underscore “_” variable in Python?

Question

What is the meaning of _ after for in this code?

if tbh.bag:
   n = 0
   for _ in tbh.bag.atom_set():
      n += 1

Answer 1

Underscore _ is considered as "I don't Care" or "Throwaway" variable in Python

• The python interpreter stores the last expression value to the special variable called _.

  >>> 10 
  10
  
  >>> _ 
  10
  
  >>> _ * 3 
  30
  

• The underscore _ is also used for ignoring the specific values. If you don’t need the specific values or the values are not used, just assign the values to underscore.

  Ignore a value when unpacking

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

  Ignore the index

  for _ in range(10):     
      do_something()