Contents

3. Types and Values

3.1 Overview

  • Python has few types. You can check the type of a variable using:

    • type(x)

  • Python uses dynamic typing (or duck typing)

    • the type is determined by the definition (if it walks like a duck, it is a duck)

  • some types are int, float, bool, str, NoneType.

3.2 String

  • In Python3, all types are classes. (so they can be referenced).

  • 'seven' and "seven" are identical in python. It’s a style choice.

  • multline strings using three single/double quotes: '''

  • x = 'seven'. x is an object.

    • x.capitalize(), x.upper(), x.lower() works.

    • x = 'seven {} {}'.format(8, 9) is another option

    • x = 'seven {1} {0}'.format(8, 9) order can be reversed

    • x = 'seven {1:<9} {0}'.format(8, 9) first arg is left aligned with 9 spaces- x = 'seven {1:<09} {0}'.format(8, 9) first arg is left aligned with 9 spaces and filled with 0s (9 is the total number of digits)

    • x = f'seven {a} {b}' f strings also work (3.6>). They use the format method.

3.3 Numeric types

  • Integer and floating are the main types. But since everything is an object, you can derive your own types.

  • 7 * 3.0 is a float.

  • 7 / 3 is also a float (new in Python 3)

  • 7 // 3 is int.

  • .1 + .1 + .1 - .3 = 5.551115123125783e-17.

    • accuracy is sacrificed for precision

    • how do you solve this?

    • from decimal import *

    • a = Decimal('0.10') …

3.4 Bool type

  • for logical values and expressions

  • None is used to represent the absence of a value

x = None
if x:
    print('True')
else:
    print('False')


prints False

  • 0, ' ' also evaluate as False

3.5 Sequence

  • Python built-in sequence types:

    • Lists

    • Tuples

    • Dictionaries

  • Lists:

    • example: x = [1, 2, 3, 4, 5]

    • list is a mutable sequence, i.e, we can reassign indices in the list. x[2] = 42.

  • Tuples:

    • example: x = (1, 2, 3, 4, 5)

    • tuples are immutable x[2] = 42 returns error

  • Range:

    • example: x = range(10) (0 to 9)

    • more general: x = range(start, stop, step)

    • to convert to list: x = list(range(10)), now it is mutable.

  • Dictionary:

    • example: `x = {‘one’: 1, ‘two’: 2}

    • to parse through keys, do for i in x:

    • to parse through key-value pairs, do for k, v in x.items():

    • dictionaries are mutable

  • Any element can be any type (and of mixed type)!

3.6 Type

  • Since everything is an object, type is the same as a class.

x = (1, 'two', 3.0, [4, 'four'], 5)
print('x is {}'.format(x))
print(type(x))

  • returns tuple

  • but type(x[1]) is a str.

  • To check type,

    • if isinstance(x, tuple)

    • if isinstance(x, list)

2. Language Overview 4. Conditionals