22 min read

Variables and Identifiers

Store values in variables, name them correctly, avoid keywords, and remember that Python is case-sensitive.

Variables

Creating and changing variablesPython

1price = 2802quantity = 33total = price * quantity4print(total)5 6price = 300          # the value can change7total = price * quantity8print(total)

Expected output

840
900

= is the assignment operator: it stores the value on the right in the variable on the left. It does not mean "equals" as in maths — read it as " becomes ". So total = total + 50 means "total becomes its old value plus 50".

Python has 35 keywords that are reserved for the language itself, including: False, None, True, and, as, break, class, continue, def, del, elif, else, except, for, from, if, import, in, is, not, or, pass, return, try, while, with.

Rules decide what is allowed; good style decides what is clear : Use meaningful names: total_price, not tp or x . Use snake_case — lower case with underscores — for variables and functions. Use CAPITALS for values that never change: DISCOUNT_RATE = 0.1 .

Two different variablesPython

1Total = 5002total = 7503print(Total, total)

Expected output

500 750

An undefined identifier — press Run to see the errorPython

1total = 7502print(Totl)

A misspelt name is a NameError : Python has never seen Totl . This is Python's version of the "undeclared identifier" error you met in the Introduction to Programming course.

Practice

Select every valid Python identifier.

Choose all that apply.

Variables

Creating and changing variablesPython

1price = 2802quantity = 33total = price * quantity4print(total)5 6price = 300          # the value can change7total = price * quantity8print(total)

Expected output

840
900

= is the assignment operator: it stores the value on the right in the variable on the left. It does not mean "equals" as in maths — read it as " becomes ". So total = total + 50 means "total becomes its old value plus 50".

Naming Rules (Identifiers)

Python has 35 keywords that are reserved for the language itself, including: False, None, True, and, as, break, class, continue, def, del, elif, else, except, for, from, if, import, in, is, not, or, pass, return, try, while, with.

Good Names

Rules decide what is allowed; good style decides what is clear : Use meaningful names: total_price, not tp or x . Use snake_case — lower case with underscores — for variables and functions. Use CAPITALS for values that never change: DISCOUNT_RATE = 0.1 .

Case Sensitivity in Action

Two different variablesPython

1Total = 5002total = 7503print(Total, total)

Expected output

500 750

An undefined identifier — press Run to see the errorPython

1total = 7502print(Totl)

A misspelt name is a NameError : Python has never seen Totl . This is Python's version of the "undeclared identifier" error you met in the Introduction to Programming course.

Practice

Select every valid Python identifier.

Choose all that apply.