Python: Learn Python in 1 Hour

By Xah Lee. Date: 2012-09-16. Last updated: 2022-08-06.

This is a Python tutorial. Spend 1 hour, and you will have a basic understanding of the language.

Examples on this page are based on Python 2.7. (python 2.x is no longer supported since year 2020)

Run Python Program

create a file with this content:

# python 2
print 1+2

Save the file as “test.py”.

To run it, go to terminal, type

python test.py

Python will print the output 3.

Or, you can run short python expressions in a command line interface. Go to terminal, type “python” to start.

python 2018-08-25 a340f — Python in terminal 2018-08-25

To exit, press Ctrl+d on a new line.

Printing

# -*- coding: utf-8 -*-
# python 2

print 3

print 3, 4

In python 2, print is a statement. You do not need parenthesis after it. It can have multiple values, separated by comma.

Strings

Strings are enclosed using 'single' quote or "double" quote.

# -*- coding: utf-8 -*-
# python 2

b = 'rabbit' # single quotes
a = "tiger"  # double quotes
print a, b   # tiger rabbit

You can use \n for linebreak, and \t for tab.

Single quote and double quote syntax have the same meaning.

Quoting Raw String r"…"

Add r in front of the string quote symbol for raw string. This way, backslash characters will NOT be interpreted as escapes. (“r” for “raw”)

c = r"this\n and that"
print c # prints a single line

Triple Quotes for Multi-Line String

To quote a string of multiple lines, use triple quotes like this '''…''' or """…""".

d = """this
will be printed
in 3 lines"""

print d

For detail, see: Python: Quote String

Unicode in String or Source File

If anywhere in your source code file contains Unicode characters, the first or second line should be:

# -*- coding: utf-8 -*-

Any string containing Unicode characters should have “u” prefix, for example, u"i ♥ cats".

# -*- coding: utf-8 -*-
# python 2

a = u"I ♥ cats" # string with unicode heart ♥

For detail, see: Python: Unicode Tutorial 🐍 .

Substring

string[begin_index:end_index]: Return a substring of string with index begin_index to end_index.

Index starts at 0.
The returned string does not include end_index.
Index can be negative, which counts from the end.

# -*- coding: utf-8 -*-
# python 2

print "01234567"[1:4] # 123

# -*- coding: utf-8 -*-
# python 2

b="01234567"

print b[1:4]  # 123
print b[1:-1] # 123456
print b[-2:-1] # 6

String Length

len(str): Return the number of chars in is string str.

print len("abc") # 3

String Join

Join string: string + string.

print "abc" + " xyz"  # "abc xyz"

String Repeat

String can be repeated using *.

print "ab" * 3 # "ababab"

String Methods

Arithmetic

# -*- coding: utf-8 -*-
# python 2

print 3 + 4 # 7
print 3 - 4 # -1
print 3 + - 4   # -1
print 3 * 4 # 12

Division, Quotient, Remainder (mod)

# -*- coding: utf-8 -*-
# python 2

# quotient
# dividing two integers is integer
print 11 / 5    # 2

# quotient with a float number
print 11 / 5.   # 2.2

# integer part of quotient
print 11 // 5   # 2
print 11 // 5.  # 2.0

# remainder, modulo
print 11 % 5    # 1
print 11 % 5.   # 1.0

# quotient and remainder
print divmod(11, 5) # (2, 1)

#  quotient and remainder
print divmod(11, 5.)  # (2.0, 1.0)

Warning: in Python 2, 11/5 returns 2, not 2.2. Use float 11/5..

Power, Root

# -*- coding: utf-8 -*-
# python 2

# power
print 2 ** 3 # 8

# square root
print 8 **(1/3.) # 2.0

In Python, power is **. The ^ is used for bitwise xor. [see Python: Operators]

Convert to {int, float, string}

Python doesn't automatically convert between {int, float, string}.

Convert to int: int(3.2).
Convert to float: float(3).
Convert to string, use repr(123) or string.format(…) method. For example, "integer {:d}, float {:f}".format(3, 3.2). [see Python: Format String]
You can write with a dot after the number as float, like this: 3..

Assignment

# -*- coding: utf-8 -*-
# python 2

# add and assign
c = 0
c += 1
print c # 1

# substract and assign
c = 0
c -= 2
print c # -2

# multiply and assign
c = 2
c *= 3
print c # 6

# power and assign
c = 3
c **= 2
print c # 9

# divide and assign
c = 7
c /= 2
print c    # 3    Note: not 3.5

# modulus (remainder) and assign
c = 13
c %= 5
print c # 3

# quotient and assign
c = 13
c //= 5
print c # 2

Note: Python doesn't support ++ or --.

Warning: ++i may not generate any error, but it doesn't do anything.

For bitwise and other operators, see: Python: Operators .

True and False

True and non-empty things are true. Number 1 is true.
False, None, zero, and empty things are false.

# -*- coding: utf-8 -*-
# python 2

my_thing = []

if my_thing:
    print "yes"
else:
    print "no"
# prints no

[see Python: True, False (boolean)]

Conditional: if then else

# -*- coding: utf-8 -*-
# python 2

x = -1
if x < 0:
    print 'neg'

# -*- coding: utf-8 -*-
# python 2

x = -1
if x < 0:
    print 'negative'
else:
    print '0 or positive'

# -*- coding: utf-8 -*-
# python 2

# Examples of if

x = -1
if x<0:
    print 'neg'
elif x==0:
    print 'zero'
elif x==1:
    print 'one'
else:
    print 'other'

# the elif can be omitted.

Loop, Iteration

while loop.

# -*- coding: utf-8 -*-
# python 2

x = 1
while x < 9:
    print x
    x += 1

for loop.

# -*- coding: utf-8 -*-
# python 2

# creates a list from 1 to 3. (does NOT include 4)
a = range(1,4)

for x in a:
    print x

The range(m, n) function gives a list from m to n, not including n.

Python also supports break and continue to exit loop.

break: Exit loop.
continue: Skip code and start the next iteration.

# -*- coding: utf-8 -*-
# python 2

for x in range(1,9):
    print 'yay:', x
    if x == 5:
        break

List

Creating a list.

a = [0, 1, 2, "more", 4, 5, 6]
print a

Counting elements:

a = ["more", 4, 6]
print len(a) # prints 3

Getting a element. Use the syntax list[index]. Index start at 0. Negative index counts from right. Last element has index -1.

a = ["more", 4, 6]
print a[1] # prints 4

Extracting a sequence of elements (aka sublist, slice): list[start_index:end_index].

# -*- coding: utf-8 -*-
a = [ "b0", "b1", "b2", "b3", "b4", "b5", "b6"]
print a[2:4]   # → ['b2', 'b3']

WARNING: The extraction does not include the element at the end index. For example, myList[2:4] returns only 2 elements, not 3.

Modify element: list[index] = newValue

# -*- coding: utf-8 -*-

a = ["b0", "b1", "b2"]
a[2] = "two"
print a # → ['b0', 'b1', 'two']

A slice (continuous sequence) of elements can be changed by assigning to a list directly. The length of the slice need not match the length of new list.

# -*- coding: utf-8 -*-
# python 2

xx = [ "b0", "b1", "b2", "b3", "b4", "b5", "b6"]
xx[0:6] = ["two", "three"]
print xx # ['two', 'three', 'b6']

Nested Lists. Lists can be nested arbitrarily. Append extra bracket to get element of nested list.

# -*- coding: utf-8 -*-
bb = [3, 4, [7, 8]]
print bb   # [3, 4, [7, 8]]
print bb[2][1]   # 8

List Join. Lists can be joined with plus sign.

b = ["a", "b"] + [7, 6]
print b # prints ['a', 'b', 7, 6]

Python Data Structure

Tuple

Python has a “tuple” type. It's like list, except that the elements cannot be changed, nor can new element added.

Syntax for tuble is using round brackets () instead of square brackets. The brackets are optional when not ambiguous, but best to always use them.

# -*- coding: utf-8 -*-
# python 2

# tuple
t1 = (3, 4 , 5)  # a tuple of 3 elements
print t1    # (3, 4 , 5)
print t1[0] # 3

# nested tuple
t2 = ((3,8), (4,9), ("a", 5, 5))
print t2[0] # (3,8)
print t2[0][0]    # 3

# a list of tuples
t3 = [(3,8), (4,9), (2,1)]
print t3[0] # (3,8)
print t3[0][0]    # 3

[see Python: Tuple]

Python Sequence Types

In Python, {string, list, tuple} are called “sequence types”. Here's example of operations that can be used on sequence type.

# length
ss = [0, 1, 2, 3, 4, 5, 6]
print len(ss)    # 7

# ith item
ss = [0, 1, 2, 3, 4, 5, 6]
print ss[0] # 0

# slice of items
ss = [0, 1, 2, 3, 4, 5, 6]
print ss[0:3]   # [0, 1, 2]

# slice of items with jump step
ss = [0, 1, 2, 3, 4, 5, 6]
print ss[0:10:2]    # [0, 2, 4, 6]

# check if a element exist
ss = [0, 1, 2, 3, 4, 5, 6]
print 3 in ss   # True. (or False)

# check if a element does NOT exist
ss = [0, 1, 2, 3, 4, 5, 6]
print 3 not in ss   # False

# concatenation
ss = [0, 1]
print ss + ss   # [0, 1, 0, 1]

# repeat
ss = [0, 1]
print ss * 2  # [0, 1, 0, 1]

# smallest item
ss = [0, 1, 2, 3, 4, 5, 6]
print min(ss)   # 0

# largest item
ss = [0, 1, 2, 3, 4, 5, 6]
print max(ss)   # 6

# index of the first occurrence
ss = [0, 1, 2, 3, 4, 5, 6]
print ss.index(3)   # 3

# total number of occurrences
ss = [0, 1, 2, 3, 4, 5, 6]
print ss.count(3)   # 1

Python Data Structure

Dictionary: Key/Value Pairs

A keyed list in Python is called “dictionary” (known as Hash Table or Associative List in other languages). It is a unordered list of pairs, each pair is a key and a value.

# -*- coding: utf-8 -*-
# python 2

# define a keyed list
aa = {"john":3, "mary":4, "joe":5, "vicky":7}
print "aa is:", aa

# getting value from a key
print "mary is:", aa["mary"]

# add a entry
aa["pretty"] = 99
print "added pretty:", aa

# delete a entry
del aa["vicky"]
print "deleted vicky", aa

# get just the keys
print "just keys", aa.keys()
# to get just values, use “.values()”

# check if a key exists
print "is mary there:", aa.has_key("mary")

Python Data Structure

Enumerate List/Dictionary

Here is a example of going thru a list by element.

myList=['one', 'two', 'three', 'infinity']
for x in myList:
     print x

You can loop thru a list and get both {index, value} of a element.

myList=['one', 'two', 'three', 'infinity']
for i, v in enumerate(myList):
     print i, v

# 0 one
# 1 two
# 2 three
# 3 infinity

The following construct loops thru a dictionary, each time assigning both keys and values to variables.

myDict = {'john':3, 'mary':4, 'joe':5, 'vicky':7}
for k, v in myDict.iteritems():
     print k, ' is ', v

[see Python: Map, Filter, List]

Module and Package

A library in Python is called a module. A collection of module is called a package.

To load a module, call import moduleName. Then, to use a function in the module, use moduleName.functionName(…).

# -*- coding: utf-8 -*-
# python 2

# import the standard module named os
import os

# example of using a function
print 'current dir is:', os.getcwd()

# -*- coding: utf-8 -*-
# python 2

import os

# print all names exported by the module
print dir(os)

[see Python: List Modules, List Loaded Modules]

Defining a Function

The following is a example of defining a function.

def myFun(x,y):
    """myFun returns x+y."""
    result = x+y
    return result

print myFun(3,4)    # 7

The string immediately following the first line is the function's documentation.

A function can have named optional parameters. If no argument is given, a default value is assumed.

def myFun(x, y=1):
    """myFun returns x+y.
    Parameter y is optional and default to 1"""
    return x+y

print myFun(3,7)    # 10
print myFun(3)  # 4

[see Python: Function]

Class and Object

Python: Class and Object

Writing a Module

Here's a basic example. Save the following line in a file and name it mm.py.

def f3(n): return n+1

To load the file, use import import mm. To call the function, use mm.f3.

import mm # import the module
print mm.f3(5)  # calling its function. prints 6
print mm.__name__   # list its functions and variables

[see Python: How to Write a Module]