NOTE: If you would like some Python development done, my company, Stochastic Technologies, is available for consulting.
Also, this tutorial is available as a short ebook.
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.
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" >>> mylist[-1] = 3.14 >>> mydict = {"Key 1": "Value 1", 2: 3, "pi": 3.14} >>> mydict["pi"] = 3.15 >>> 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. 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]
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 supports Unicode out of the box, using the syntax u"This is a unicode string". 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: Poromenos 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.
Flow control statements
Flow control statements are
if
, for
, and while
. There is no select
; instead, use if. Use for to enumerate through members of a list. To obtain a list of numbers, use range(<number>)
. These statements' syntax is thus:rangelist = range(10) >>> print rangelist [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] for number in rangelist: # 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: pass
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*. 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 f(x): return x + 1 functionvar = lambda x: x + 1 >>> print functionvar(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 at least two leading underscores and at most one trailing one (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 classes. >>> 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", "w") 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") loadedlist = pickle.load(myfile) myfile.close() >>> print loadedlist ['This', 'is', 4, 13327]
Miscellaneous
>>> 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 an item has a specific property. # "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 how many items have this property. >>> sum(1 for i in [3, 3, 4, 4, 3] if i == 4) 2 >>> del lst1[0] >>> print lst1 [2, 3] >>> del lst1
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).
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