Identifiers

In programming languages, identifiers are the names used to identify a variable, a function, or any other entity in a program. Choosing them is not free-form — Python lays down precise rules.

Rules for naming an identifier in Python

• The name must begin with an uppercase letter, a lowercase letter, or an underscore _. After the first character, any combination of the characters a–z, A–Z, 0–9 and underscore may follow. A direct consequence: an identifier cannot start with a digit — marks1 is legal, 1marks is not.
• It can be of any length, though it is preferred to keep identifiers short and meaningful.
• It must not be a keyword (any reserved word listed in Table 5.1).
• Special symbols are not allowed anywhere in an identifier — characters such as !, @, #, the dollar sign, % and the like cannot appear in a name.

Why meaningful names matter

Identifiers are for the humans reading the program as much as for the interpreter. Consider computing the average of a student's marks in three subjects. Choosing the identifiers marks1, marks2, marks3 and avg — rather than a, b, c or A, B, C — makes the statement self-explanatory:

avg = (marks1 + marks2 + marks3)/3

Similarly, to compute the area of a rectangle, names such as area, length and breadth give clarity and readability that single letters cannot:

area = length * breadth

Checking the rules against examples