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This tutorial is available as a short ebook. The e-book features extra content from follow-up posts on various Python best practices, all in a convenient, self-contained format. All future updates are free for people who purchase it.
Preliminary fluff
So, you want to learn the Python programming language but can’t find a concise and yet full-featured tutorial. This tutorial will attempt to teach you Python in 10 minutes. It’s probably not so much a tutorial as it is a cross between a tutorial and a cheatsheet, so it will just show you some basic concepts to start you off. Obviously, if you want to really learn a language you need to program in it for a while. I will assume that you are already familiar with programming and will, therefore, skip most of the non-language-specific stuff. The important keywords will be highlighted so you can easily spot them. Also, pay attention because, due to the terseness of this tutorial, some things will be introduced directly in code and only briefly commented on.
We will focus on Python 3, as that is the version you should use. All the examples in the book are in Python 3, and if anyone advises you to use 2, they aren’t your friend.
Properties
Python is strongly typed (i.e. types are enforced), dynamically, implicitly typed (i.e. you don’t have to declare variables), case sensitive (i.e. var and VAR are two different variables) and object-oriented (i.e. everything is an object).
Getting help
Help in Python is always available right in the interpreter. If you want to know
how an object works, all you have to do is call help(<object>)!
Also useful are dir(), which shows you all the object’s methods, and
<object>.__doc__, which shows you its documentation string:
>>> help(5)
Help on int object:
(etc etc)
>>> dir(5)
['__abs__', '__add__', ...]
>>> abs.__doc__
'abs(number) -> number
Return the absolute value of the argument.
Syntax
Python has no mandatory statement termination characters and blocks are specified by indentation. Indent to begin a block, dedent to end one. Statements that expect an indentation level end in a colon (:). Comments start with the pound (#) sign and are single-line, multi-line strings are used for multi-line comments. Values are assigned (in fact, objects are bound to names) with the equals sign (“=”), and equality testing is done using two equals signs (“==”). You can increment/decrement values using the += and -= operators respectively by the right-hand amount. This works on many datatypes, strings included. You can also use multiple variables on one line. For example:
>>> myvar = 3
>>> myvar += 2
>>> myvar
5
>>> myvar -= 1
>>> myvar
4
"""This is a multiline comment.
The following lines concatenate the two strings."""
>>> mystring = "Hello"
>>> mystring += " world."
>>> print(mystring)
Hello world.
# This swaps the variables in one line(!).
# It doesn't violate strong typing because values aren't
# actually being assigned, but new objects are bound to
# the old names.
>>> myvar, mystring = mystring, myvar
Data types
The data structures available in python are lists, tuples and dictionaries.
Sets are available in the sets library (but are built-in in Python 2.5 and
later). Lists are like one-dimensional arrays (but you can also have lists of
other lists), dictionaries are associative arrays (a.k.a. hash tables) and
tuples are immutable one-dimensional arrays (Python “arrays” can be of any type,
so you can mix e.g. integers, strings, etc in lists/dictionaries/tuples). The
index of the first item in all array types is 0. Negative numbers count from the
end towards the beginning, -1 is the last item. Variables can point to
functions. The usage is as follows:
>>> sample = [1, ["another", "list"], ("a", "tuple")]
>>> mylist = ["List item 1", 2, 3.14]
>>> mylist[0] = "List item 1 again" # We're changing the item.
>>> mylist[-1] = 3.21 # Here, we refer to the last item.
>>> mydict = {"Key 1": "Value 1", 2: 3, "pi": 3.14}
>>> mydict["pi"] = 3.15 # This is how you change dictionary values.
>>> mytuple = (1, 2, 3)
>>> myfunction = len
>>> print(myfunction(mylist))
3
You can access array ranges using a colon (:). Leaving the start index empty
assumes the first item, leaving the end index assumes the last item. Indexing is
inclusive-exclusive, so specifying [2:10] will return items [2] (the third
item, because of 0-indexing) to [9] (the tenth item), inclusive (8 items).
Negative indexes count from the last item backwards (thus -1 is the last item)
like so:
>>> mylist = ["List item 1", 2, 3.14]
>>> print(mylist[:])
['List item 1', 2, 3.1400000000000001]
>>> print(mylist[0:2])
['List item 1', 2]
>>> print(mylist[-3:-1])
['List item 1', 2]
>>> print(mylist[1:])
[2, 3.14]
# Adding a third parameter, "step" will have Python step in
# N item increments, rather than 1.
# E.g., this will return the first item, then go to the third and
# return that (so, items 0 and 2 in 0-indexing).
>>> print(mylist[::2])
['List item 1', 3.14]
Strings
Its strings can use either single or double quotation marks, and
you can have quotation marks of one kind inside a string that uses
the other kind (i.e. “He said ’hello’.” is valid). Multiline
strings are enclosed in _triple double (or single) quotes_ (“”“).
Python strings are always Unicode, but there is another string type
that is pure bytes. Those are called bytestrings and are represented
with the b prefix, for example b'Hello \xce\xb1'.
. To fill a string with values, you use the %
(modulo) operator and a tuple. Each %s gets replaced with an item
from the tuple, left to right, and you can also use dictionary
substitutions, like so:
>>> print("Name: %s\
Number: %s\
String: %s" % (myclass.name, 3, 3 * "-"))
Name: Stavros
Number: 3
String: ---
strString = """This is
a multiline
string."""
# WARNING: Watch out for the trailing s in "%(key)s".
>>> print("This %(verb)s a %(noun)s." % {"noun": "test", "verb": "is"})
This is a test.
>>> name = "Stavros"
>>> "Hello, {}!".format(name)
Hello, Stavros!
>>> print(f"Hello, {name}!")
Hello, Stavros!
Flow control statements
Flow control statements are if, for, and while. There is no switch;
instead, use if. Use for to enumerate through members of a list. To obtain
a sequence of numbers you can iterate over, use range(<number>). These
statements’ syntax is thus:
>>> print(range(10))
range(0, 10)
>>> rangelist = list(range(10))
>>> print(rangelist)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
for number in range(10):
# Check if number is one of
# the numbers in the tuple.
if number in (3, 4, 7, 9):
# "Break" terminates a for without
# executing the "else" clause.
break
else:
# "Continue" starts the next iteration
# of the loop. It's rather useless here,
# as it's the last statement of the loop.
continue
else:
# The "else" clause is optional and is
# executed only if the loop didn't "break".
pass # Do nothing
if rangelist[1] == 2:
print("The second item (lists are 0-based) is 2")
elif rangelist[1] == 3:
print("The second item (lists are 0-based) is 3")
else:
print("Dunno")
while rangelist[1] == 1:
print("We are trapped in an infinite loop!")
Functions
Functions are declared with the def keyword. Optional arguments are
set in the function declaration after the mandatory arguments by being assigned
a default value. For named arguments, the name of the argument is assigned
a value. Functions can return a tuple (and using tuple unpacking you can
effectively return multiple values). Lambda functions are ad hoc
functions that are comprised of a single statement. Parameters are passed by
reference, but immutable types (tuples, ints, strings, etc) cannot be changed
in the caller by the callee. This is because only the memory location of the
item is passed, and binding another object to a variable discards the old
one, so immutable types are replaced. For example:
# Same as def funcvar(x): return x + 1
funcvar = lambda x: x + 1
>>> print(funcvar(1))
2
# an_int and a_string are optional, they have default values
# if one is not passed (2 and "A default string", respectively).
def passing_example(a_list, an_int=2, a_string="A default string"):
a_list.append("A new item")
an_int = 4
return a_list, an_int, a_string
>>> my_list = [1, 2, 3]
>>> my_int = 10
>>> print(passing_example(my_list, my_int))
([1, 2, 3, 'A new item'], 4, "A default string")
>>> my_list
[1, 2, 3, 'A new item']
>>> my_int
10
Classes
Python supports a limited form of multiple inheritance in classes. Private
variables and methods can be declared (by convention, this is not enforced by
the language) by adding a leading underscore (e.g. _spam). We can also bind
arbitrary names to class instances. An example follows:
class MyClass(object):
common = 10
def __init__(self):
self.myvariable = 3
def myfunction(self, arg1, arg2):
return self.myvariable
# This is the class instantiation
>>> classinstance = MyClass()
>>> classinstance.myfunction(1, 2)
3
# This variable is shared by all instances.
>>> classinstance2 = MyClass()
>>> classinstance.common
10
>>> classinstance2.common
10
# Note how we use the class name
# instead of the instance.
>>> MyClass.common = 30
>>> classinstance.common
30
>>> classinstance2.common
30
# This will not update the variable on the class,
# instead it will bind a new object to the old
# variable name.
>>> classinstance.common = 10
>>> classinstance.common
10
>>> classinstance2.common
30
>>> MyClass.common = 50
# This has not changed, because "common" is
# now an instance variable.
>>> classinstance.common
10
>>> classinstance2.common
50
# This class inherits from MyClass. The example
# class above inherits from "object", which makes
# it what's called a "new-style class".
# Multiple inheritance is declared as:
# class OtherClass(MyClass1, MyClass2, MyClassN)
class OtherClass(MyClass):
# The "self" argument is passed automatically
# and refers to the class instance, so you can set
# instance variables as above, but from inside the class.
def __init__(self, arg1):
self.myvariable = 3
print(arg1)
>>> classinstance = OtherClass("hello")
hello
>>> classinstance.myfunction(1, 2)
3
# This class doesn't have a .test member, but
# we can add one to the instance anyway. Note
# that this will only be a member of classinstance.
>>> classinstance.test = 10
>>> classinstance.test
10
Exceptions
Exceptions in Python are handled with try-except [exceptionname] blocks:
def some_function():
try:
# Division by zero raises an exception
10 / 0
except ZeroDivisionError:
print("Oops, invalid.")
else:
# Exception didn't occur, we're good.
pass
finally:
# This is executed after the code block is run
# and all exceptions have been handled, even
# if a new exception is raised while handling.
print("We're done with that.")
>>> some_function()
Oops, invalid.
We're done with that.
Importing
External libraries are used with the import [libname] keyword. You can also
use from [libname] import [funcname] for individual functions. Here is an
example:
import random
from time import clock
randomint = random.randint(1, 100)
>>> print(randomint)
64
File I/O
Python has a wide array of libraries built in. As an example, here is how
serializing (converting data structures to strings using the pickle
library) with file I/O is used:
import pickle
mylist = ["This", "is", 4, 13327]
# Open the file C:\\binary.dat for writing. The letter r before the
# filename string is used to prevent backslash escaping.
myfile = open(r"C:\\binary.dat", "wb")
pickle.dump(mylist, myfile)
myfile.close()
myfile = open(r"C:\\text.txt", "w")
myfile.write("This is a sample string")
myfile.close()
myfile = open(r"C:\\text.txt")
>>> print(myfile.read())
'This is a sample string'
myfile.close()
# Open the file for reading.
myfile = open(r"C:\\binary.dat", "rb")
loadedlist = pickle.load(myfile)
myfile.close()
>>> print(loadedlist)
['This', 'is', 4, 13327]
Miscellaneous
- Conditions can be chained:
1 < a < 3checks that a is both less than 3 and greater than 1. - You can use
delto delete variables or items in arrays. - List comprehensions provide a powerful way to create and manipulate lists.
They consist of an expression followed by a
forclause followed by zero or moreiforforclauses, like so:
>>> lst1 = [1, 2, 3]
>>> lst2 = [3, 4, 5]
>>> print([x * y for x in lst1 for y in lst2])
[3, 4, 5, 6, 8, 10, 9, 12, 15]
>>> print([x for x in lst1 if 4 > x > 1])
[2, 3]
# Check if a condition is true for any items.
# "any" returns true if any item in the list is true.
>>> any([i % 3 for i in [3, 3, 4, 4, 3]])
True
# This is because 4 % 3 = 1, and 1 is true, so any()
# returns True.
# Check for how many items a condition is true.
>>> sum(1 for i in [3, 3, 4, 4, 3] if i == 4)
2
>>> del lst1[0]
>>> print(lst1)
[2, 3]
>>> del lst1
- Global variables are declared outside of functions and can be read without
any special declarations, but if you want to write to them you must declare them
at the beginning of the function with the
globalkeyword, otherwise Python will bind that object to a new local variable (be careful of that, it’s a small catch that can get you if you don’t know it). For example:
number = 5
def myfunc():
# This will print 5.
print(number)
def anotherfunc():
# This raises an exception because the variable has not
# been bound before printing. Python knows that it an
# object will be bound to it later and creates a new, local
# object instead of accessing the global one.
print(number)
number = 3
def yetanotherfunc():
global number
# This will correctly change the global.
number = 3
Epilogue
This tutorial is not meant to be an exhaustive list of all (or even a subset) of Python. Python has a vast array of libraries and much much more functionality which you will have to discover through other means, such as the excellent book Dive into Python. I hope I have made your transition in Python easier. Please leave comments if you believe there is something that could be improved or added or if there is anything else you would like to see (classes, error handling, anything).
By the way, you should follow me on Twitter.
