This is a basic tutorial on Python. The goal is to get a quick working understanding of the language. Examples on this page are based on Python 2.7. For python 3, see: Python 3 Basics.
Strings are enclosed using single quote or double quote. example
# -*- coding: utf-8 -*- # python a = "tiger" b = 'rabbit' # single/double quotes are basically the same in Python print a, b # prints 「tiger rabbit」
You can use \n for linebreak, and \t for tab, etc.
To quote a string of multiple lines, use triple quotes. Example:
d = """this will be printed in 3 lines""" print d
You can add r in front of the quote symbol. This way, backslash characters will NOT be interpreted as as escapes. (“r” for “raw”)
c = r"this\n and that" print c # prints a single line
more detail: Python, Ruby, Perl: Quoting Strings
If your string contains Unicode, or anywhere in your source file, the first or second line should be # -*- coding: utf-8 -*-. I recommend to always start your file with this.
Also, any string containing Unicode should have “u” prefix u"…".
# -*- coding: utf-8 -*- # python a = u"I ♥ Emacs" # string with unicode heart 「♥」
Substring extraction is done by appending a bracket [‹begin index›:‹end index›]. Index can be negative, which counts from the end.
# -*- coding: utf-8 -*- # python b="01234567" print b[1:4] # prints “123”
Length of the string is len().
a="this" print len(a) # 4
Strings can be joined by a plus sign +.
print "this" + " that"
String can be repeated using *.
print "this" * 2
Python, Ruby, Perl: Basic String Operations
# -*- coding: utf-8 -*- # python print(3 + 4) # 7 print(3 - 4) # -1 print(3 + - 4) # -1 print(3 * 4) # 12 print(2 ** 3) # 8 power print(11 / 5) # 2 (quotient) print(11 / 5.) # 2.2 (force to float) print(11 // 5) # 2 (quotient) print(11 % 5) # 1 remainder (modulo) print(divmod(11, 5)) # (2, 1) quotient and remainder print(divmod(11.2, 5)) # (2.0, 1.1999) quotient and remainder
In Python, power is “**”. The “^” is used for bitwise xor. 〔☛ Python 3: Operators〕
Warning: in Python 2, 11/5 returns 2, not 2.2.
Python doesn't automatically convert between {int, float, string}.
int(3.2)float(3)."int %d, float %f".format(3, 3.2). Detail: Python, Ruby, Perl: Formatting Strings
3..# -*- coding: utf-8 -*- # Python # 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 # exponent and assign c = 3 c **= 2 print c # 9 # divide and assign c = 7 c /= 2 print c # 3 note: note 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 3: Operators.
The following evaluates to false:
False. A builtin Boolean type.None. A builtin type.0. Zero.0.0. Zero, float.Empty string and Empty list-like things are all false.
"". Empty string.[]. Empty list.(). Empty tuple.{}. Empty dictionary.set([]). Empty set.frozenset([]). Empty frozen set.# -*- coding: utf-8 -*- # python my_thing = [] if my_thing: print "yes" else: print "no"
#-*- coding: utf-8 -*- # python x = -1 if x < 0: print 'neg'
#-*- coding: utf-8 -*- # python x = -1 if x < 0: print 'negative' else: print '0 or positive'
#-*- coding: utf-8 -*- # python # 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.
# -*- coding: utf-8 -*- # python a = range(1,5) # creates a list from 1 to 4. (does NOT include the end range) for x in a: if x % 2 == 0: print x, 'even'
In the above, the percent “%” symbol calculates the remainder of division. The range(m,n) function gives a list from m to n-1.
Python also supports “break” and “continue” to exit the loop. “break” will exit the loop. “continue” will skip code and start the next iteration. Example of using “break”.
#-*- coding: utf-8 -*- # python for x in range(1,9): print 'yay:', x if x == 5: break
#-*- coding: utf-8 -*- # python x = 1 while x < 9: print x x += 1
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 sequences of elements (aka sublist, slice): ‹myList›[‹startIndex›:‹endIndex›].
a = ["nil", "uni", "bi", "tri", "quad", "quint", "sex"] print a[2:4] # prints ["bi", "tri"]
WARNING: The extraction is not inclusive. For example, ‹myList›[2:4] returns only 2 elements, not 3.
Modify elements: ‹myList›[‹index›] = ‹new value›.
a = ["nilpotent", "unisex", "bisexual"] a[2] = "two" print a # prints ['nilpotent', 'unisex', '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 a = ["nilpotent", "unisex", "bisexual", "tribadism", "quadriceps", "quintessence", "sex"] a[0:6] = ["two", "three"] print a # prints ['two', 'three', 'sex']
Nested Lists. Lists can be nested arbitrarily. Append extra bracket to get element of nested list.
a = [3, 4, [7, 8]] print a print a[2][1] # returns 8
List Join. Lists can be joined with plus sign.
b = ["a", "b"] + [7, 6] print b # prints ['a', 'b', 7, 6]
Python, Ruby, Perl: List Basics
Python has a “tuple” type. It's like list, except it's immutable (that is, the elements cannot be changed).
Syntax for tuble is using round brackets instead of square brackets.
# -*- coding: utf-8 -*- # python # 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
〔☛ Python: What's the Difference Between Tuple and List?〕
In Python, {string, list, tuple} are called “sequence types”. They all have the same methods. Here's example of operations that can be used on sequence type.
# -*- coding: utf-8 -*- # python # operations on sequence types # a list ss = [0, 1, 2, 3, 4, 5, 6] # length print len(ss) # 7 # ith item print ss[0] # 0 # slice of items print ss[0:3] # [0, 1, 2] # slice of items with jump step print ss[0:10:2] # [0, 2, 4, 6, 8] # check if a element exist print 3 in ss # True. (or False) # check if a element does NOT exist print 3 not in ss # False # concatenation print ss + ss # [0, 1, 2, 3, 4, 5, 6, 0, 1, 2, 3, 4, 5, 6] # repeat print ss * 2 # [0, 1, 2, 3, 4, 5, 6, 0, 1, 2, 3, 4, 5, 6] # smallest item print min(ss) # 0 # largest item print max(ss) # 6 # index of the first occurence print ss.index(3) # 3 # total number of occurences print ss.count(3) # 1
A keyed list in Python is called “dictionary”. It is a unordered list of pairs, each consists of a key and a value.
#-*- coding: utf-8 -*- # python # define a keyed list aa = {"john":3, "mary":4, "jane":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")
http://docs.python.org/lib/typesmapping.html
See: Python ＆ Perl: Dictionary, Hash
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. Example:
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, 'jane':5, 'vicky':7}
for k, v in myDict.iteritems():
print k, ' is ', v
See also: Python, Ruby, Perl: Apply a Function to a List.
A library in Python is called a module.
# -*- coding: utf-8 -*- # python # import the standard module named os import os # example of using a function print 'current dir is:', os.getcwd()
# -*- coding: utf-8 -*- # python import os # print all names exported by the module print dir(os)
More: Python ＆ Perl: Using Modules/Packages/Library.
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) # prints 7
A string immediately following the function definition is the function's documentation.
A function can have optional parameters. If no argument is given, a default value is assumed. Example:
def myFun(x, y=1): """myFun returns x+y. Parameter y is optional and default to 1""" return x+y print myFun(3)
For defining infinite number of parameters, or unspecified keyword parameters, see: Python, Ruby, Perl: Defining A Function.
Example:
# -*- coding: utf-8 -*- # python # in the following, we define a set of data and functions as a class, # and name it xxx class xxx: "A class extempore! =(^_^)=" i=1 # i'm a piece of data def okaydokey(self): return "okaydokey" def square(self,a): return a*a # in the following, we create a object of the class xxx. This is # called “instantiating a class”. x = xxx() # Data or functions defined in a class are called the class's # attributes or methods. To use them, append a dot and their name # after the object's name. print 'value of attribute i is:', x.i print "3 squared is:", x.square(3) print "okaydokey called:", x.okaydokey() # In the definition of function inside a class, the first parameter # “self” is necessary. It is just side-effect of the language design. # The first line in the class definition is the class's # documentation. It can be accessed thru the __doc__ attribute. print "xxx's doc string is:", x.__doc__ # var inside the class can be change like this x.i = 400 # new data can be added to the class x.j=4 print x.j # A class's method can also be over-rided x.square = 333 # (the following line will no longer work) ## print "3 squared is:", x.square(3)
Here's a basic example. Save the following line in a file and name it 〔mymodule.py〕.
def f1(n): return n+1
To load the file, use import import mymodule, then to call the function, use moduleName.functionName. Example:
import mymodule # import the module print mymodule.f1(5) # calling its function print mymodule.__name__ # list its functions and variables
http://docs.python.org/lib/typesmodules.html