Python Tutorial

If the user wants to store the value of last expression in the interpreter.
If the user wants to declare the variable or function in the code.
If the user wants to separate the digits of number lateral value.
For ignoring some values.
Underscore is also used as 'localization (l10n)' and 'Internationalization (i18n)' functions.

Let's see some examples of these cases.

Using in Interpreter

The interpreter of Python will store the last expression value in the '_'.

Example:

print(47)
47
print(_)
47
print(_ + 13)
60
print(_)
60
print(_ - 10)
50

For Ignoring the Values

Here, we will use Underscore (_) for ignoring the values.

Example:

Here,

	A = 10
	B = 20
	C = 30
	D = 40
	E = 60
	F = 70
	G = 80

And underscore (_) is used for ignoring value 50.

Using Underscore (_) to Declare the Variable and Function

Python language does not support private variables or functions, so, the users cannot force private variables and function of somethings but they can call them directly from the other modules.

Example:

The users can use the trailing underscore (_) for avoiding the conflicts with Python keywords and built-ins.

Example:

The above double underscore (__) is used for avoiding the conflicts with attributes names between the names.

If the users are writing the function name "__display" in the class, the name will then be modified in "_ClassName__display" form.

Example:

In some cases, users use this form as __init__.

For Separating Digits of Number Lateral Value

The underscore (_) can be used for separating the digits of number lateral value

Example:

decoder_base = 5_000_000
print (decoder_base)

Output:

As Localization (l10n) and International (i18n) functions

This is just a principle and not any syntactic function. The underscore (_) is used for binding the i18n/l10n to the variable with underscore. Its concept is taken from C convention.

Underscore Prefix with Python Variables Means?

The names in the class, which have leading underscore (_) are for indicating to the other users that the attribute or function is intended to be private in the program.

The users are recommended that they should use single underscore (_) for semiprivate and double underscore (__) for fully private variables.

In the following example, we will show the difference between Single Underscore (_) and Double Underscore (__) prefixes.

Example:

class UserClass():
     def __init__(self):
             self.__fully_private = "JavaTpoint"
             self.__fully_private = "to"
             self._semi_private = "Hello"
             self._semi_private = "Say"
mc = UserClass()
print(mc._semi_private)
print(mc.__fully_private)

Output:

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-10-4f3b78146b9f> in <module>
      7 mc = UserClass()
      8 print(mc._semi_private)
----> 9 print(mc.__fully_private)

AttributeError: 'UserClass' object has no attribute '__fully_private'

In the above code, the user tried to access the fully private variables and in output, they will get the error saying that the UserClass does not have any attribute called '__fully_private'.