Variables and expressions


The most commonly used command in Python is assignment. The Python code to create a Vairiable called x that is to be given the value π is:

>>> pi = 3.14159

In Python, unlike many other programming languages, neither a variable declaration nor an end-of-line delimiter is necessary. The line is terminated by the end of the line. Variables are created automatically when they are assigned for the first time.


In Python, variables are labels that refer to objects. Any number of labels can refer to the same object, and if that object changes, so does the value to which all those variables refer. To better understand what this means, see the following example:

>>> x = [1, 2, 3]
>>> y = x
>>> y[0] = 4
>>> print(x)
[4, 2, 3]

However, variables can also refer to constants:

>>> x = 1
>>> y = x
>>> z = y
>>> y = 4
>>> print(x,y,z)
1 4 1

In this case, after the third line, x, y and z all refer to the same immutable integer object with the value 1. The next line, y = 4, causes y to refer to the integer object 4, but this does not change the references of x or z.

Python variables can be set to any object, whereas in many other languages variables can only be stored in the declared type.

Variable names are case-sensitive and can contain any alphanumeric character as well as underscores, but must begin with a letter or underscore.


If you receive a SyntaxError, check whether the variable name is a keyword. Keywords are reserved for use in Python language constructs, so you cannot turn them into variables. After calling help() you can enter keywords to get the keywords:

>>> help()
help> keywords
Here is a list of the Python keywords.  Enter any keyword to get more help.
False               class               from                or
None                continue            global              pass
True                def                 if                  raise
and                 del                 import              return
as                  elif                in                  try
assert              else                is                  while
async               except              lambda              with
await               finally             nonlocal            yield
break               for                 not


You can use a variable name to overwrite built-in functions, types and other objects so that they can then only be accessed via the builtins module. These variable names should therefore never be used. You can obtain a list of the __builtins__ objects with :

>>> dir(__builtins__)
['ArithmeticError', 'AssertionError', 'AttributeError', 'BaseException', 'BaseExceptionGroup', 'BlockingIOError', 'BrokenPipeError', 'BufferError', 'BytesWarning', 'ChildProcessError', 'ConnectionAbortedError', 'ConnectionError', 'ConnectionRefusedError', 'ConnectionResetError', 'DeprecationWarning', 'EOFError', 'Ellipsis', 'EncodingWarning', 'EnvironmentError', 'Exception', 'ExceptionGroup', 'False', 'FileExistsError', 'FileNotFoundError', 'FloatingPointError', 'FutureWarning', 'GeneratorExit', 'IOError', 'ImportError', 'ImportWarning', 'IndentationError', 'IndexError', 'InterruptedError', 'IsADirectoryError', 'KeyError', 'KeyboardInterrupt', 'LookupError', 'MemoryError', 'ModuleNotFoundError', 'NameError', 'None', 'NotADirectoryError', 'NotImplemented', 'NotImplementedError', 'OSError', 'OverflowError', 'PendingDeprecationWarning', 'PermissionError', 'ProcessLookupError', 'RecursionError', 'ReferenceError', 'ResourceWarning', 'RuntimeError', 'RuntimeWarning', 'StopAsyncIteration', 'StopIteration', 'SyntaxError', 'SyntaxWarning', 'SystemError', 'SystemExit', 'TabError', 'TimeoutError', 'True', 'TypeError', 'UnboundLocalError', 'UnicodeDecodeError', 'UnicodeEncodeError', 'UnicodeError', 'UnicodeTranslateError', 'UnicodeWarning', 'UserWarning', 'ValueError', 'Warning', 'ZeroDivisionError', '__build_class__', '__debug__', '__doc__', '__import__', '__loader__', '__name__', '__package__', '__spec__', 'abs', 'aiter', 'all', 'anext', 'any', 'ascii', 'bin', 'bool', 'breakpoint', 'bytearray', 'bytes', 'callable', 'chr', 'classmethod', 'compile', 'complex', 'copyright', 'credits', 'delattr', 'dict', 'dir', 'divmod', 'enumerate', 'eval', 'exec', 'exit', 'filter', 'float', 'format', 'frozenset', 'getattr', 'globals', 'hasattr', 'hash', 'help', 'hex', 'id', 'input', 'int', 'isinstance', 'issubclass', 'iter', 'len', 'license', 'list', 'locals', 'map', 'max', 'memoryview', 'min', 'next', 'object', 'oct', 'open', 'ord', 'pow', 'print', 'property', 'quit', 'range', 'repr', 'reversed', 'round', 'set', 'setattr', 'slice', 'sorted', 'staticmethod', 'str', 'sum', 'super', 'tuple', 'type', 'vars', 'zip']


Python supports arithmetic and similar expressions. The following code calculates the average of x and y and stores the result in the variable z:

>>> x = 1
>>> y = 2
>>> z = (x + y) / 2


Arithmetic operators that use only integers do not always return an integer. As of Python 3, division returns a floating point number. If you want the traditional integer division to return an integer, you can use // instead.