An Informal Introduction to Python

Get started with Python basics including numbers, strings, and lists

Let’s dive into Python by exploring its interactive interpreter as a calculator, working with text, and manipulating lists.

In the examples below, input and output are distinguished by the presence or absence of prompts (>>> and ...). To try these examples, type everything after the prompt.

Using Python as a Calculator

Numbers

The interpreter acts as a simple calculator. You can type expressions and get results:

>>> 2 + 2
4
>>> 50 - 5*6
20
>>> (50 - 5*6) / 4
5.0
>>> 8 / 5  # division always returns a float
1.6

Integer division and remainder:

>>> 17 / 3   # classic division returns a float
5.666666666666667
>>> 17 // 3  # floor division discards the fractional part
5
>>> 17 % 3   # the % operator returns the remainder
2
>>> 5 * 3 + 2  # floored quotient * divisor + remainder
17

Powers:

>>> 5 ** 2  # 5 squared
25
>>> 2 ** 7  # 2 to the power of 7
128

Variables

Use the equal sign (=) to assign values:

>>> width = 20
>>> height = 5 * 9
>>> width * height
900

Trying to use an undefined variable raises a NameError:

>>> n  # try to access an undefined variable
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'n' is not defined

The Special _ Variable

In interactive mode, the last printed expression is assigned to _:

>>> tax = 12.5 / 100
>>> price = 100.50
>>> price * tax
12.5625
>>> price + _
113.0625
>>> round(_, 2)
113.06

Treat _ as read-only. Don’t assign to it explicitly, as this creates a new local variable that masks the built-in behavior.

Text (Strings)

Python can manipulate text using the str type. Strings can be enclosed in single or double quotes:

>>> 'spam eggs'  # single quotes
'spam eggs'
>>> "Paris rabbit got your back :)!"  # double quotes
'Paris rabbit got your back :)!'
>>> '1975'  # digits in quotes are also strings
'1975'

Escaping Quotes

To include quotes in strings, escape them with \ or use the other quote type:

>>> 'doesn\'t'  # escape the single quote
"doesn't"
>>> "doesn't"  # or use double quotes
"doesn't"
>>> '"Yes," they said.'
'"Yes," they said.'

Special Characters

The print() function produces more readable output:

>>> s = 'First line.\nSecond line.'  # \n means newline
>>> s  # without print(), special characters are shown
'First line.\nSecond line.'
>>> print(s)  # with print(), \n produces a new line
First line.
Second line.

Raw Strings

Use raw strings (prefix with r) to prevent backslash interpretation:

>>> print('C:\some\name')  # \n creates a newline!
C:\some
ame
>>> print(r'C:\some\name')  # note the r before the quote
C:\some\name

Multi-line Strings

Use triple quotes for strings spanning multiple lines:

>>> print("""\
... Usage: thingy [OPTIONS]
...      -h                        Display this usage message
...      -H hostname               Hostname to connect to
... """)
Usage: thingy [OPTIONS]
     -h                        Display this usage message
     -H hostname               Hostname to connect to

String Operations

Concatenation and repetition:

>>> 3 * 'un' + 'ium'
'unununium'

Automatic concatenation of literals:

>>> 'Py' 'thon'
'Python'
>>> text = ('Put several strings within parentheses '
...         'to have them joined together.')
>>> text
'Put several strings within parentheses to have them joined together.'

This only works with literals, not variables:

>>> prefix = 'Py'
>>> prefix 'thon'  # can't concatenate a variable and a literal
SyntaxError: invalid syntax
>>> prefix + 'thon'  # use + for variables
'Python'

Indexing and Slicing

Strings can be indexed (subscripted):

>>> word = 'Python'
>>> word[0]  # character at position 0
'P'
>>> word[5]  # character at position 5
'n'
>>> word[-1]  # last character
'n'
>>> word[-2]  # second-last character
'o'

Slicing to get substrings:

>>> word[0:2]  # characters from position 0 (included) to 2 (excluded)
'Py'
>>> word[2:5]  # characters from position 2 (included) to 5 (excluded)
'tho'
>>> word[:2]   # from the beginning to position 2 (excluded)
'Py'
>>> word[4:]   # from position 4 (included) to the end
'on'
>>> word[-2:]  # from second-last (included) to the end
'on'

Visual representation:

 +---+---+---+---+---+---+
 | P | y | t | h | o | n |
 +---+---+---+---+---+---+
 0   1   2   3   4   5   6
-6  -5  -4  -3  -2  -1

String Immutability

Strings cannot be changed (they’re immutable):

>>> word[0] = 'J'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'str' object does not support item assignment

Create a new string instead:

>>> 'J' + word[1:]
'Jython'

String Length

>>> s = 'supercalifragilisticexpialidocious'
>>> len(s)
34

Lists

Lists are compound data types that group together values:

>>> squares = [1, 4, 9, 16, 25]
>>> squares
[1, 4, 9, 16, 25]

Indexing and Slicing Lists

Like strings, lists support indexing and slicing:

>>> squares[0]  # indexing returns the item
1
>>> squares[-1]
25
>>> squares[-3:]  # slicing returns a new list
[9, 16, 25]

List Concatenation

>>> squares + [36, 49, 64, 81, 100]
[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

List Mutability

Unlike strings, lists are mutable:

>>> cubes = [1, 8, 27, 65, 125]  # something's wrong here
>>> 4 ** 3  # the cube of 4 is 64, not 65!
64
>>> cubes[3] = 64  # replace the wrong value
>>> cubes
[1, 8, 27, 64, 125]

Adding Items

>>> cubes.append(216)  # add the cube of 6
>>> cubes.append(7 ** 3)  # and the cube of 7
>>> cubes
[1, 8, 27, 64, 125, 216, 343]

Assignment and References

Assignment doesn’t copy data. Multiple variables can refer to the same list. Use slicing to create a copy.

>>> rgb = ["Red", "Green", "Blue"]
>>> rgba = rgb
>>> rgba.append("Alpha")
>>> rgb
["Red", "Green", "Blue", "Alpha"]

Use slicing to create a copy:

>>> correct_rgba = rgba[:]
>>> correct_rgba[-1] = "Fixed"
>>> rgba
["Red", "Green", "Blue", "Alpha"]

Modifying Lists

Assignment to slices:

>>> letters = ['a', 'b', 'c', 'd', 'e', 'f', 'g']
>>> letters[2:5] = ['C', 'D', 'E']  # replace some values
>>> letters
['a', 'b', 'C', 'D', 'E', 'f', 'g']
>>> letters[2:5] = []  # remove them
>>> letters
['a', 'b', 'f', 'g']
>>> letters[:] = []  # clear the list
>>> letters
[]

Nested Lists

>>> a = ['a', 'b', 'c']
>>> n = [1, 2, 3]
>>> x = [a, n]
>>> x
[['a', 'b', 'c'], [1, 2, 3]]
>>> x[0]
['a', 'b', 'c']
>>> x[0][1]
'b'

First Steps Towards Programming

Let’s write a Fibonacci series using a while loop:

>>> # Fibonacci series:
>>> # the sum of two elements defines the next
>>> a, b = 0, 1
>>> while a < 10:
...     print(a)
...     a, b = b, a+b
...
0
1
1
2
3
5
8

Key features demonstrated:

1. Multiple assignment: a, b = 0, 1 assigns simultaneously
2. While loop: Executes as long as the condition is true
3. Indentation: Python uses indentation to group statements
4. Print function: Writes output to the screen

Customizing Print Output

Control the line ending with the end parameter:

>>> a, b = 0, 1
>>> while a < 1000:
...     print(a, end=',')
...     a, b = b, a+b
...
0,1,1,2,3,5,8,13,21,34,55,89,144,233,377,610,987,

Next Steps

Now that you understand Python basics, explore Control Flow Tools to learn about if statements, for loops, and functions.