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
>>> 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
>>> 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'
>>> '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'
>>> 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'
+---+---+---+---+---+---+
| 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
>>> 'J' + word[1:]
'Jython'
String Length
>>> s = 'supercalifragilisticexpialidocious'
>>> len(s)
34
>>> 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"]
>>> 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
- Multiple assignment:
a, b = 0, 1 assigns simultaneously
- While loop: Executes as long as the condition is true
- Indentation: Python uses indentation to group statements
- 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. 