Informal Intro Tutorial

# Start Python and see the >>> prompt
>>> 2 + 2
4
>>> 50 - 5*6
20
>>> (50 - 5*6) / 4
5.0
>>> 8 / 5  # Division always returns a float
1.6

# Variables and assignment
>>> width = 20
>>> height = 5 * 9
>>> width * height
900

# Different number types
>>> 17 / 3  # Classic division returns float
5.666666666666667
>>> 17 // 3  # Floor division
5
>>> 17 % 3   # Remainder
2
>>> 5 ** 2   # Power
25

# Mixed operations
>>> 4 * 3.75 - 1
14.0
>>> tax = 12.5 / 100
>>> price = 100.50
>>> price * tax
12.5625
>>> price + _  # _ holds the last printed value
113.0625

# Single or double quotes
>>> 'spam eggs'
'spam eggs'
>>> "spam eggs"
'spam eggs'
>>> 'doesn\'t'  # Escape quote with backslash
"doesn't"
>>> "doesn't"   # Or use double quotes
"doesn't"

# Multi-line strings
>>> 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

# Concatenation and repetition
>>> 'Py' + 'thon'
'Python'
>>> 3 * 'un' + 'ium'
'unununium'

# String indexing
>>> word = 'Python'
>>> word[0]   # First character
'P'
>>> word[5]   # Last character
'n'
>>> word[-1]  # Last character (negative indexing)
'n'
>>> word[-2]  # Second to last
'o'

# String slicing
>>> word[0:2]   # Characters from 0 to 2 (exclusive)
'Py'
>>> word[2:5]   # Characters from 2 to 5
'tho'
>>> word[:2]    # Beginning to 2
'Py'
>>> word[4:]    # From 4 to end
'on'
>>> word[-2:]   # Last two characters
'on'

# Creating and accessing lists
>>> squares = [1, 4, 9, 16, 25]
>>> squares
[1, 4, 9, 16, 25]
>>> squares[0]  # Indexing
1
>>> squares[-1]  # Last item
25
>>> squares[-3:]  # Last three items
[9, 16, 25]

# Lists are mutable
>>> cubes = [1, 8, 27, 65, 125]  # Oops, 4³ is 64, not 65!
>>> cubes[3] = 64  # Fix the error
>>> cubes
[1, 8, 27, 64, 125]

# Adding elements
>>> cubes.append(216)  # Add 6³
>>> cubes.append(7 ** 3)  # Add 7³
>>> cubes
[1, 8, 27, 64, 125, 216, 343]

# List slicing and assignment
>>> 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']

# Length and nesting
>>> len(letters)
4
>>> a = ['a', 'b', 'c']
>>> n = [1, 2, 3]
>>> x = [a, n]  # Nested list
>>> x
[['a', 'b', 'c'], [1, 2, 3]]
>>> x[0]
['a', 'b', 'c']
>>> x[0][1]
'b'

# Fibonacci series up to n
>>> a, b = 0, 1  # Multiple assignment
>>> while a < 10:
...     print(a)
...     a, b = b, a+b  # Multiple assignment
...
0
1
1
2
3
5
8

# The loop body is indented!
# Indentation groups statements together
>>> i = 256*256
>>> print('The value of i is', i)
The value of i is 65536

# Comments start with #
>>> # This is a comment
... 2 + 2  # And a comment on the same line
4

# Using variables
>>> message = "Hello, World!"
>>> print(message)
Hello, World!

# Everything is an object
>>> name = "Python"
>>> print(f"Hello, {name}!")  # f-string (modern way)
Hello, Python!

# Length of strings and lists
>>> len("Hello")
5
>>> len([1, 2, 3, 4, 5])
5

# Simple conditionals (more detail in later tutorials)
>>> x = int(input("Please enter an integer: "))
Please enter an integer: 42
>>> if x < 0:
...     x = 0
...     print('Negative changed to zero')
... elif x == 0:
...     print('Zero')
... elif x == 1:
...     print('Single')
... else:
...     print('More')
...
More

# For loops with lists
>>> words = ['cat', 'window', 'defenestrate']
>>> for w in words:
...     print(w, len(w))
...
cat 3
window 6
defenestrate 12

# Common functions you'll use often
>>> list(range(5))
[0, 1, 2, 3, 4]
>>> list(range(5, 10))
[5, 6, 7, 8, 9]
>>> list(range(0, 10, 3))
[0, 3, 6, 9]

>>> sum([1, 2, 3, 4, 5])
15
>>> max([1, 2, 3, 4, 5])
5
>>> min([1, 2, 3, 4, 5])
1

# Type checking
>>> type(42)
<class 'int'>
>>> type("hello")
<class 'str'>
>>> type([1, 2, 3])
<class 'list'>