Assignment evaluates a value and binds a name to it. In Python, variables are created automatically on first assignment, so declarations, type annotations, and semicolons are unnecessary. Names are case-sensitive, may contain letters, digits, and underscores, and must begin with a letter or underscore. section 4.3 section 4.0
A Python variable is better understood as a label referring to an object than as a container holding a value. Several names can refer to the same mutable object, so changing that object is visible through every alias. Reassigning one name instead points that name elsewhere. section 4.3
x = 5
The basic assignment form creates x and associates it with the integer 5. section 4.3
Shared objects versus reassignment
With a mutable list, assigning b = a makes both names refer to the same list; changing b[1] changes what a and c observe. With an immutable integer, b = 5 reassigns only b, leaving a unchanged. section 4.3
Assignment also supports several practical operations:
- Delete a name with del; using it afterward raises NameError. section 4.3
- Assign multiple values at once by packing the right-hand values and unpacking them into names on the left. section 5.7.3
- Swap values concisely with var1, var2 = var2, var1 instead of using a temporary variable. section 5.7.3
- Use a starred target to collect surplus values into a list; it receives an empty list when no surplus remains. section 5.7.3
x = (1, 2, 3, 4) a, b, *c = x a, b, c (1, 2, [3, 4]) a, *b, c = x a, b, c (1, [2, 3], 4) *a, b, c = x a, b, c ([1, 2], 3, 4) a, b, c, d, *e = x a, b, c, d, e (1, 2, 3, 4, [])
Extended unpacking shows how a starred target absorbs the unmatched middle or outer values and produces a list. section 5.7.3
