Valid Identifiers

Every variable and constant needs a name, called an identifier. Python only accepts identifiers that follow its rules, so a name that breaks them is a syntax error and the program will not run at all.

An identifier may contain letters, digits and underscores. It cannot contain spaces, cannot start with a digit, and cannot be a Python keyword such as if, for or while. Keywords are words Python reserves for its own instructions, so Python rejects them as names.

Identifier	Valid?	Reason
`bus_number`	Valid	Uses letters and an underscore, and starts with a letter.
`bus number`	Invalid	Contains a space.
`2nd_score`	Invalid	Starts with a digit.
`while`	Invalid	Is a Python keyword.

Identifiers are case-sensitive: total, Total and TOTAL are three separate names, and creating one does not create the others. You must also spell a name consistently. If you assign a value to lap_time, then write laptime later, Python treats the second name as a new, undefined identifier rather than the one you meant.

The same rules apply whether the name is for a variable or a constant.

score = 15
print(Score)

Output:

NameError: name 'Score' is not defined (line 2)

Practise this