Python: Variables

Variables (1)

A variable is a name for … something

  • Something has a type

  • … but its name hasn’t

>>> a = 42
>>> type(a)
<class 'int'>
>>> a = 1.5
>>> type(a)
<class 'float'>
>>> a = [42,'blah']
>>> type(a)
<class 'list'>
../../../../_images/0140-variables-datatypes-basic.svg

Variables (2)

Python is a “dynamic language” (whatever that means)

  • Names have no type

  • Created when first assigned

  • Runtime error when accessed but not yet there

  • … as opposed to compiled languages (whatever that means)

Naming rules: just like most other languages

  • Start with Letters (Unicode since Python 3, ASCII in Python 2) or underscore

  • Numbers in the following characters

  • Case sensitive

Assignment Fun

Multiple assignments in one statement
a, b, c = 1, "Eins", 1.0
a, b = b, a  # "swap"
  • Tuple unpacking

  • Important concept throughout the entire language

  • ⟶ later

Assignment has a value
a = b = c = 1
  • Assignment is right associative

  • a, b, c are assigned 1

Assignment Details

More than one ever wants to know …

  • Day-to-day programming does not need to know

  • Good to know when something goes wrong

  • Only valid for immutable types (int, float, str)

a = 42
b = a
b = 7

Or equivalently …:

a = 42
b = 42
b = 7
../../../../_images/0140-variables-datatypes-assignment-details.svg