Why Variable Names Should Not Start with Digits: Language Rules, Readability, and Lexical Analysis

Revisiting the Definition of Variable

Variables are identifiers used to store and represent data, and a variable name is the identifier that can be referenced in code.

Python Naming Rules

The first character must be a letter from the alphabet or an underscore _ ;

The remaining characters may consist of letters, digits, and underscores;

Identifiers are case‑sensitive.

In Python 3, non‑ASCII identifiers such as Chinese characters are allowed.

Human Confusion

If a variable could start with a digit, the following code would be ambiguous:

0x756b5b3 = 0x756b5b3

Here a hexadecimal literal is mistakenly used as a variable name, making it hard to distinguish between the variable and the constant.

1e5 = 1e6

Using 1e5 as a variable would clash with scientific‑notation literals, leading to confusion about whether the token is a variable or a numeric constant.

Computer Confusion

Lexical analyzers treat any token that begins with a digit as a numeric literal; allowing digit‑leading identifiers would force the lexer to perform additional checks, reducing efficiency and increasing complexity.

Summary

Although a few languages permit identifiers that start with digits, it is generally considered poor practice because it can be confused with numeric constants and harms code readability and maintainability.

Easter Egg

Consider the following absurd code snippet:

1 = 2

if 1 == 2:
  print("1 等于 2 吗？")
else:
  print("2 等于 2 吗？")

The example illustrates how allowing digit‑leading identifiers would lead to nonsensical and error‑prone programs.