Why can't python variables start with a number?

As long as something is stipulated in writing, there must be a reason for it, that is, it must be reasonable.

Python variable naming rules(Recommended learning: Python video tutorial)

Variable names can only contain Letters, numbers and underscores. Variable names can start with a letter or an underscore, but not a number. For example, you can name a variable message_1, but you cannot name it 1_message.

Variable names cannot contain spaces, but underscores can be used to separate words. For example, the variable name greeting_message works, but the variable name greeting message raises an error.

Do not use Python keywords and function names as variable names, that is, do not use words that Python reserves for special purposes, such as print.

Variable names should be short and descriptive. For example, name is better than n, student_name is better than s_n, name_length is better than length_of_persons_name.

Use the lowercase letter l and the uppercase letter O with caution, as they may be mistaken for numbers 1 and 0;

Note: Lowercase Python variable names should be used. Although using uppercase letters in variable names will not cause errors, it is a good idea to avoid using uppercase letters.

So why can't it start with a number? This should be explained from the perspective of compilation principles.

This is what I suddenly thought of when I was doing script analysis today. Let’s first explain our grammar parsing tool. The script to be parsed is very simple and only includes operators such as &, |, ~, =, !=, () and {}. At the beginning, the compilation principle was adopted, drawing NFA and DFA, finding the status, and then writing lexical analysis to generate the Token. Then the syntax analysis generated the Token to generate a syntax tree based on the semantics, and finally evaluated it. Thinking about it later, this script is very simple. It only needs to judge character by character and enter different sub-functions for processing when encountering different types of characters. At the same time, it completes the process of lexical analysis, syntax analysis and evaluation. However, no matter which method is used, it is necessary to determine which Token is currently entered based on the characters read in. This is the crux of the problem.

Why is it critical to determine which Token a character belongs to?

Suppose we cancel the restriction that variable names cannot start with numbers. When the lexical analyzer enters the initial state of a Token analysis, if the first character read is a number, Then the lexical analyzer cannot determine whether the Token it is currently analyzing is a variable name or a numeric constant. Well, if you say that the analyzer can judge based on the following characters, then if the next character is a letter, then it is easy to determine that the current Token belongs to the variable name (we will ignore the reserved keywords for the time being), but if the next What should I do if the characters are all numbers? The parser will not be able to tell because numbers are allowed in the definition of variable names.

Haha, what I said above may be convoluted and difficult to understand. In simple words: when analyzing the string "123", if the variable name allows the first character to be a number, the analyzer I don't know whether "123" is a numerical constant or a variable name.

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