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Python Identifiers

In this tutorial, we will explore:

  • What identifiers are
  • Rules for defining identifiers
  • Valid and invalid examples
  • Naming conventions (underscore usage)

What is an Identifier?​

In real life, we identify people using names. These names help us distinguish one person from another.

In Python​

An identifier is a name used to identify variables, functions, classes, or other objects in a Python program.

What Can Be an Identifier?​

An identifier can be:

  • Variable name
  • Function (method) name
  • Class name
  • Module name

Example​

a = 10
  • a is the identifier
  • It is used to represent the value 10

Another Example​

class Test:
    pass
  • Test is a class name
  • It is also an identifier

Rules for Defining Python Identifiers​

Just like naming a person involves some thought and convention, Python also has strict rules for naming identifiers.

Rule 1: Allowed Characters​

Only the following characters are allowed:

  • Alphabets (a–z, A–Z)
  • Digits (0–9)
  • Underscore (_)

Valid Examples​

cash = 10
total123 = 20
my_var = 30

Invalid Examples​

ca$h = 10     
@value = 20

Rule 2: Cannot Start with a Digit​

An identifier must not begin with a number.

# following is valid
total123 = 10

# following is invalid
123total = 10   # ❌ Invalid

Rule 3: Case Sensitivity​

Python is a case-sensitive language, which means, Uppercase and lowercase letters are treated differently.

Example​

total = 10
Total = 20
TOTAL = 30

These are three different variables.

Rule 4: No Length Limit​

Python does not impose any limit on identifier length.

Example​

this_is_a_very_long_variable_name_used_for_demo_purpose = 10
print(this_is_a_very_long_variable_name_used_for_demo_purpose)

Rule 5: Cannot Use Reserved Words​

Python has some predefined words called keywords.

Examples:

  • if
  • else
  • for
  • while

Invalid Example​

if = 10   # ❌ Invalid

Valid Example​

value = 10

Special Meaning of Underscore (_)​

Identifiers using underscores have special conventions in Python.

1. Normal Variable​
x = 10
2. Protected Variable (Single Underscore)​
_x = 10
  • Convention: intended for internal use
  • Indicates "protected" usage
3. Private Variable (Double Underscore)​
__x = 10
  • Stronger restriction
  • Used mainly in classes (name mangling)
4. Magic Variables (Double Underscore Both Sides)​
__init__
__name__
  • Special predefined variables
  • Used internally by Python