Indentation:
Whitespace is important in Python. Actually, whitespace at the beginning of the line is important. This is called indentation. Leading whitespace (spaces and tabs) at the beginning of the logical line is used to determine the indentation level of the logical line, which in turn is used to determine the grouping of statements. This means that statements which go together must have the same indentation. Each such set of statements is called a block. Program in a bracketed language would merely become more difficult to read and consumes much time of the programmer and also affects the readability. Indentation solves the above problems of a bracketed languages (like C, C++ etc).
points to be noted while using indentation:
- Do not use a mixture of tabs and spaces for the indentation as it does not work across different platforms properly.
- Better to use any one of the popular indentation styles and most important is to use it consistently.
example code using a 'four space' indentation:
def foo(x): if x == 0: bar() baz() else: qux(x) foo(x - 1)
Python Syntax
Basic Script
###############################################################################
# name: main
# purpose: entry point of the script
###############################################################################
# sys.argv is a list of command line argument strings (pos 0 is script name)
if __name__ == "__main__":
print "hello world" # if no comma (,) at end \n is auto included
Data Types
###############################################################################
# name: main
# purpose: show the basic datatypes
###############################################################################
### scalars (bools, ints, floats, strings)
pi = 3.14 # can hold real / whole numbers
false0 = False # can hold boolean values (False / True)
false1 = 0 # 0 counts as False, non-zero is True
hello = "hello" # can hold strings (immutable)
false2 = "" # en empty string is False of course
false2 = None # false2 not defined anymore, counts as False
line = "hello\n" # line is hello and then a new-line (LF) - 6 chars
firm = r"hello\n" # line is hello\n - 7 chars
quot1 = "a\"b'c" # quot1 is a"b'c
quot2 = 'a"b\'c' # quot2 is a"b'c
multiline = # long text with a few lines
"""This is long text.
With several lines."""
# scalar operations
num = num*2 + 3 - myfloat # num is 23.86
num = 2**4 % 5 # num is 1 - exp then modulus
num += 1 # num is 2 - c-like syntax
m = (1<<3)&0xff|0x03^0x01 # m is 0x0a
strop = 'he'+'lo'*3 # strop is 'helololo'
print hello[0] # prints 'h', [-1] is 'o', hello[0] = 'j' is illegal
print hello[0:2] # prints 'he', [2:] is 'llo', [:3] is 'hel'
### lists and tuples (sequences of scalars)
nums = [1,2,3] # lists are mutable and initialized with []
mixed = [pi,"three",3.13] # any scalar can be placed in a list
empty = [] # empty list counts as False, non-empty is True
[one,two] = [1,2] # one is 1, two is 2
tup1 = (1,2,3) # tuples are immutable (fast) and initialized with ()
tup2 = (pi,"three",3.13) # any scalar can be placed in a tuple
empty_tuple = () # empty tuple counts as False, non-empty is True
single_tuple = ("a",) # tuple with 1 member must have a trailing comma (,)
(one,two) = (-1,-2) # one is -1, two is -2
# list and tuple operations
first = nums[0] # first is 1, [-1] is 3 (lists/tuples)
sl = nums[0:2] # sl is [1,2], [2:] is [3], [:1] is [1] (lists/tuples)
mixed[1] = "neo" # mixed is [3.14,'neo',3.13] (lists only)
mixed[0:2] = ["a","b"] # mixed is ['a','b',3.13] (lists only)
joined = nums*3 + ["a",9] # joined is [1,2,3,1,2,3,1,2,3,'a',9] (lists/tuples)
nested = [nums,8] # nested is [[1,2,3],8]
length = len(nums) # len() function returns length (lists/tuples)
bool = 1 in nums # bool is True since 1 is nums[0] (lists/tuples)
t = tuple(nums) # tuple turns lists into tuples, list does opposite
new = [x*2 for x in nums] # new is [2,4,6], this is called list comprehensions
nums.extend([4,5]) # nums is [1,2,3,4,5], lists have methods (lists only)
dir(nums) # view all methods of a list - object (lists only)
### dictionaries (hashes of keys with values)
dict1 = {10:"ten","key":"val"} # key 10 with val "ten", "key" with val "val"
ages = {"jim":18,"ted":21} # any type can be used as a key or value
empty_dict = {} # empty hash counts as False, non-empty is True
# dictionary operations
val = dict1[10] # val is "ten"
ages["ron"] = 24 # key "ron" with value 24 added to ages
ages["ron"] += 1 # key "ron" just incremented to 25 (a key has one val)
del ages["ron"] # key "ron" with its value are removed from ages
b = ages.has_key("billy") # b is False, dicts have methods (use dir() to see)
### sets (a list without regard to order, when every member appears once)
set1 = set(["c","a","c"]) # the list will not contain duplicates anymore
set2 = set("abba") # will create a set of unique letters
# set operations
bool = "a" in set1 # bool is True, fast membership testing
set3 = set1 - set2 # members is set1 but not in set2 ["c"]
set3 = set1 | set2 # members in either set1 or set2 ["a","b","c"]
set3 = set1 & set2 # members in both set1 and set2 ["a"]
set3 = set1 ^ set2 # members in set1 or set2 but not both ["b","c"]
Conditionals
###############################################################################
# name: main
# purpose: show the basic conditionals
###############################################################################
# regular c style if statement
if (value is not None) and (value == 1): # is tests for same object
print "value equals 1",
print " more can come in this block"
# if-else block
if (job == "millionaire") or (state != "dead"): # !=,== can be used for strings
print "a suitable husband found" # <> for lexographical compare
else:
print "not suitable"
list1 = (1,2,3,9)
list2 = (1,2,7)
# comparing lists / tuples compares var by var until one is different
if (list1 <= list2) and (not age < 80):
print "1 = 1, 2 = 2, but 3 <= 7 so its True"
# short form with no blocks is also ok
if ok: print "ok"
# since there is no switch statement, this is the shortest way to reproduce
if (i == 1): do_something1()
elif (i == 2): do_something2()
elif (i == 3): do_something3()
else: do_something4()
Loops and Iterations
###############################################################################
# name: main
# purpose: show the flow blocks
###############################################################################
# for statement (iterate on members of a sequence)
sentence = ['Marry','had','a','little','lamb']
for word in sentence: # word holds a member in each iteration
print word, len(word)
# simulate a c for statement
for i in range(10): # range(3) returns [0,1,2]
print i
for i in xrange(1000): # xrange() does not allocate all before
print i # more efficient for large ranges
# while statements are very similar to c
i = 0
while (i < 10): # enter block if condition is TRUE
print "iteration number ",i
i += 1
# short forms (no blocks needed if a single statement comes before)
for i in xrange(10): print i
while not end_of_file(): read_next_line()
# continue and break statements are similar to c
for i in xrange(10):
if i == 3: continue # skip printing 3 (go to next iteration)
if i == 5: break # exit the loop before printing 5
print i, # will print 0 1 2 4
# pass statement does nothing but required when statement syntactically needed
while True:
pass # loop endlessly until keyboard interrupt
Functions
###############################################################################
# name: main
# purpose: show function and subroutine syntax
###############################################################################
# return values
def print_hello(): ### does not return anything (actually returns None)
print "hello"
def seventeen(): ### returns the number 17
return 17
num = seventeen() + 53 # function call
def retlist(): ### all datatypes can be returned
return [1,2,3]
[one,two,thr] = retlist() # function call
# arguments
def has_args(arg1,arg2): ### all types can be passed as args
num = arg1 + 4
mylist = arg2 + ['a',7]
has_args(5.16,[1,'b']) # function call
def sqrt(num,debug=False): ### some arguments can have default values
if debug: print "Sqrt:" # defaults are evaluated once so don't use mutables
return calc_sqrt(num)
sqrt(9) # function call without default arguments
sqrt(16,True) # function call with default arguments
sqrt(25,debug=True) # function call with named arguments (supply middle)
def many(num,**keywords): ### keywords is a hash of all named arguments given
print "num is ",num # num is regular mandetory arg
print "named args are:"
print keywords.keys()
many(1) # function call with no named args, keywords is {}
many(2,debug=True) # function call with, keywords is {'debug':True}
def arg_list(num,*arglist): ### arglist is a tuple of the arguments
print "num is ",num # num is regular mandetory arg
print "arg list is:"
print arglist
arg_list(1) # function call with no extra list, arglist is ()
arg_list(2,'a',3.14) # function call with, arglist is ('a',3.14)
args = [3,6] # other way around is also possible (give a list)
r = range(*args) # calls range(3,6), r is [3,4,5]
# lambda form (on the fly functions)
def duplicate_n_maker(n): ### this function creates a function
return lambda arg1:arg1*n
dup3 = duplicate_n_maker(3) # dup3() is a new function
dup_str = dup3('go') # dup_str is 'gogogo'
Exceptions
###############################################################################
# name: main
# purpose: show basic exception and error handling
###############################################################################
# needed for file ops and exc_info()
import sys
# simple exception usage
try: # wrap "dangerous" code in a try block
f = open("file.txt")
except IOError: # catch IOError and deriving exceptions
print "Could not open"
else: # optional, run block if no exception thrown
f.close()
# a bit more complex usage
a = [1,2,3] # init some simple list
try:
a[7] = 0 # will throw an IndexError exception
except (IndexError,TypeError):# handle several types with (,) syntax
print "IndexError caught" # handle the exception
except Exception, e: # catch all deriving from Exception (instance e)
print "Exception: ", e # address the instance, print e.__str__()
except: # catch everything
print "Unexpected:" # handle unexpected exceptions
print sys.exc_info()[0] # info about curr exception (type,value,traceback)
raise # re-throw caught exception
try:
a[7] = 0 # will throw an IndexError exception
finally: # cleanup code that should always run (no except)
print "Will run regardless" # will run if thrown, not thrown, caught, uncaught
# implement a very simple personalized exception class
class MyException(Exception): # derive from builtin Exception class (not a must)
"""My general exception with an output message."""
def __init__(self,msg): # implement a ctor with one mandatory argument
self.msg = msg # place the msg as a member on this class
def __str__(self): # implement so instance could be printed directly
return repr(self.msg) # builtin repr() func returns string rep of objs
# implement the simplest personalized exception class possible
class MyMiniEx(Exception): # use Exception's __init__, __str__, etc
pass
# play with our personalized exceptions
try:
raise MyException("hello") # must supply argument since __init__ requires it
except MyException, e: # catch just thrown MyException with instance e
print e # run implemented __str__ function
try:
raise MyMiniEx # arguments here are optional (as in Exception)
except Exception: # catch MyMiniEx since derives from Exception
pass # do nothing
Classes
###############################################################################
# name: MyVector
# purpose: implement a simple class
###############################################################################
class MyVector:
"""A simple vector class."""
num_created = 0 # static member, initialized to 0
def __init__(self,x=0,y=0): # ctor, no overloading possible so only one
self.__x = x # create members by placing variables on self
self.__y = y # __ variable prolog indicated private variable
MyVector.num_created += 1 # update the static variable
def get_size(self): # all methods must accept the self argument
return self.__x+self.__y
@staticmethod # static methods added only in new versions
def get_num_created(): # with decorator @staticmethod (no self needed)
return MyVector.num_created
# usage
print MyVector.num_created # access static variables
v = MyVector() # create a vector with default values
w = MyVector(0.23,0.98) # create a vector with given values
print w.get_size() # call a member on an instance
bool = isinstance(v, MyVector) # check if v is an instance of MyVector, True
###############################################################################
# name: MyObject
# purpose: implement a simple base class (which Soap will derive from)
###############################################################################
class MyObject:
def __init__(self,weight): # simple ctor with a mandatory argument
self.__weight = weight # init private member __weight
def print_name(self): pass # this will be overridden, all methods virtual
def get_weight(self): # another method
return self.__weight
class Soap(MyObject): # derives from MyObject, multiple is ok
"""A throwable branded soap."""
def __init__(self,brand): # override the ctor
MyObject.__init__(self,1.33) # must always call base ctor manually
self.__brand = brand
def print_name(self): # override the base method with same name
print "Soap produced by ", self.__brand,
print "which weighs ", self.get_weight()
def __del__(self): # dtor, called by garbage collector when freed
print "No more soap" # similar to java finalize, do not rely on it
MyObject.__del__(self) # must manually call base dtor if exists
# usage
s = Soap("Gillette") # create a soap with given value
m = s.get_weight() # call base method (in MyObject)
s.print_name() # call overriden method (in Soap)
bool = isinstance(s, MyObject) # check if s is an instance of MyObject, True
del s # manually decrease reference count (optional)
###############################################################################
# name: CustomizedVector
# purpose: override some special methods for vector customization
###############################################################################
class MyVector: # continued class from above
def __repr__(self): # called by repr() func for string representation
return "MyVector("+repr(self.__x)+","+repr(self.__y)+")"
def __str__(self): # called by print() when obj is printed
return "vector with coords ("+repr(self.__x)+","+repr(self.__y)+")"
def __cmp__(self,other): # obj comparison (<>==), retval similar to strcmp
return (self.get_size() - other.get_size())
def __nonzero__(self): # bool() truth value testing
if (self.get_size()>0): return True
else: return False
def __add__(self,other): # implement + operator for object
return MyVector(self.__x+other.__x,self.__y+other.__y)
# usage
print repr(v)
print w
if w>v: print "w is bigger"
if not v: print "v is False"
print v+w
Interactive Interpreter
###############################################################################
# name: main
# purpose: show interpreter oriented functions and internal help system
###############################################################################
# entering an object in the interpreter prints it value
>>> lst = [1,2,3] # init a list
>>> lst # prints on console: [1, 2, 3]
# var _ is the last returned object
>>> _ # prints on console: [1, 2, 3]
# get a list of varible names, function names, module names in a namespace
dir() # names defined in current namespace (main)
dir(__builtins__) # module __builtins__ includes all builtin functions
dir(sys) # module sys provides access to operating system calls
dir(lst) # methods and members of list object named lst
# document a class or a function
def sqrt(n): # one liners (the doc string has to be first in the func)
"""Calc the square root of a number."""
def sqrt(n): # or more elaborate descriptions (blank line by convention)
"""Calc the square root of a number.
Arguments:
n - Number to calc square root of.
Returns:
Square root of n.
"""
print sqrt.__doc__ # doc string accessible through object __doc__ member
# use the internal help system
help(sqrt) # print the doc string of a given object
help(help) # print basic help about the help command
help(sys) # prints long documentation of the sys module
help() # start the interactive help system
Regular Expressions
###############################################################################
# name: main
# purpose: show regular expression usage
###############################################################################
# needed for regular expressions
import re
# matching (actually searching since match checks beginning only)
c = 'Someone, call 911.' # the string we want to match upon
mo = re.search(r'call',c) # mo is a match obj instance (or None)
s = mo.group(0) # s is 'call' - entire matched string
t = mo.span(0) # t is (9,13) - tuple of (start,end) pos
mo = re.search(r'Some(...)',c) # mo is a match obj instance (or None)
s = mo.group(1) # s is 'one' - mo.group(0) is 'Someone'
t = mo.groups() # t is ('one') - tuple of mo.group from 1 up
t = mo.span(1) # t is (4,7) - mo.span(0) is (0,7)
# global matching (get all found, like /g in perl)
i = re.finditer(r'.o.',c) # i is an iterator of all mo found
for mo in i: print mo.group(0) # will print 'Som' 'eon'
l = re.findall(r'.o.',c) # l is ['Som','eon'] - without mo, just strs
l = re.findall(r'o(.)(.)',c) # l is [('m','e'),('n','e')] - groups are ok
# substituting
g = "hello world" # the string we want to replace in
g = re.sub(r'hello','world',g) # g is now 'goodbye world'
# splitting
l = re.split(r'\W+',c) # l is ['Someone','call','911','']
l = re.split(r'(\W+)',c) # l is ['Someone',', ','call',' ','911','.','']
# pattern syntax (to make things short g0 is retval.group(0), g1 is group(1))
re.search(r'c.11',c) # . is anything but \n, g0 is 'call'
re.search(r'c.11',c,re.S) # S is singe-line, . will include \n, g0 is 'call'
re.search(r'911\.',c) # \ escapes metachars {}[]()^$.|*+?\, g0 is '911.'
re.search(r'o..',c) # matches earliest, g0 is 'ome'
re.search(r'g?one',c) # ? is 0 or 1 times, g0 is 'one'
re.search(r'cal+',c) # + is 1 or more times, g0 is 'call', * for 0 or more
re.search(r'cal{2}',c) # {2} is exactly 2 times, g0 is 'call'
re.search(r'cal{0,3}',c) # {0,3} is 0 to 3 times, g0 is 'call', {2,} for >= 2
re.search(r'S.*o',c) # matches are greedy, g0 is 'Someo'
re.search(r'S.*?o',c) # ? makes match non-greedy, g0 is 'So'
re.search(r'^.o',c) # ^ must match beginning of line, g0 is 'So'
re.search(r'....$',c) # $ must match end of line, g0 is '911.'
re.search(r'9[012-9a-z]',c)# one of the letters in [...], g0 is '91'
re.search(r'.o[^m]',c) # none of the letters in [^...], g0 is 'eon'
re.search(r'\d*',c) # \d is digit, g0 is '911'
re.search(r'S\w*',c) # \w is word [a-zA-Z0-9_], g0 is 'Someone'
re.search(r'..e\b',c) # \b is word boundry, g0 is 'one', \B for non-boundry
re.search(r' \D...',c) # \D is non-digit, g0 is ' call', \W for non-word
re.search(r'\s.*\s',c) # \s is whitespace char [\t\n ], g0 is ' call '
re.search(r'\x39\x31+',c) # \x is hex byte, g0 is '911'
re.search(r'Some(.*),',c) # (...) extracts, g1 is 'one', g0 is 'Someone,'
re.search(r'e(one|two)',c) # | means or, g0 is 'eone', g1 is 'one'
re.search(r'e(?:one|tw)',c)# (?:...) does not extract, g0 is 'eone', g1 is None
re.search(r'(.)..\1',c) # \1 is memory of first brackets, g0 is 'omeo'
re.search(r'some',c,re.I) # I is case-insensitive, g0 is 'Some'
re.search(r'^Some',c,re.M) # M is multi-line, ^ will match start of each line
Standard IO
###############################################################################
# name: main
# purpose: show some basic IO and file handling
###############################################################################
# copy textual lines from input file to an output file
fin = None # init fin (so cleanup will not throw)
fout = None # init fout for same reason
try: # file IO is "dangerous"
fin = open("input.txt","r") # open input.txt, mode as in c fopen
fout = open("output.txt","w") # open output.txt, mode as in c fopen
first = fin.readline() # read line with "\n" at end, "" (False) on EOF
print "First line:", line
for line in fin: # implements iterator interface (readline loop)
fout.write(line) # writes a string to a file
except IOError, e: # catch IOErrors, e is the instance
print "Error in file IO: ", e # print exception info if thrown
if fin: fin.close() # cleanup, close fin only if open (not None)
if fout: fout.close() # cleanup, close fout only if open (not None)
# read binary records from a file
from struct import * # needed for struct.unpack function
fin = None # init fin (so cleanup will not throw)
try: # file IO is "dangerous"
fin = open("input.bin","rb") # open input.bin in binary read mode
s = f.read(8) # read 8 bytes (or less) into a string
while (len(s) == 8): # continue as long as we have full records
x,y,z = unpack(">HH<L", s) # parse 2 big-end ushorts, 1 little-end ulong
print "Read record: " \ # print a long line
"%04x %04x %08x"%(x,y,z) # formatting similar to c printf
s = f.read(8) # read another record
except IOError: # catch IOErrors, no instance
pass # just continue on problem
if fin: fin.close() # cleanup, close fin only if open (not None)
# print a line without automatic new-line at end
print "without a new line",
# read entire file to memory at once
data = fin.read()
# format strings sprintf-style (% is a basic property of any str)
s = "%s has %d eyes" % (name,2)
# serialize and deserialize complex objects into files
import pickle # needed for serialization logic
ages = {"ron":18,"ted":21} # create a complex data structure
pickle.dump(ages,fout) # serialize the map into a writable file
ages = pickle.load(fin) # deserialize the map from a readable file
# navigating the file system and listing files (dir modules for more info)
import os # needed for basic OS interaction
print os.getcwd() # get current directory
os.chdir('..') # change current directory
import glob # needed for file globbing with wildcards
lst = glob.glob('*.txt') # get a list of files according to wildcard
import shutil # needed for file management tasks
shutil.copyfile('a.py','a.bak') # copy a file from source to destination
Useful Functions and Modules
###############################################################################
# name: main
# purpose: show some basic functions and useful modules
# notes: dir() and help() modules to navigate them from the interpreter
###############################################################################
# builtin functions (module __builtins__)
b = len('hello') > len([1,2,3]) # b is True since 5 > 3 (returns length)
print 'a is',chr(ord('a')) # ord converts chr to num, chr is opposite
i = int('38')+int('0x3a',16) # i is 96 (integer), long() func similar
print repr(96) # repr is a formal (eval) string rep
print str(96) # str is for user rep (optional in print)
print hex(58) # prints '0x3a'
l = map(hex,[10,20]) # run hex() on list, l is ['0xa','0x14']
l = range(0,10,3) # create lists, l is [0,3,6,9]
i = reduce(lambda x,y:x+y,[1,2,3,4]) # runs func as list reduces 10=((1+2)+3)+4
# import a module (echo) from a package (Sound.Effects) to current namespace
import Sound.Effects.echo # in code: Sound.Effects.echo.echofilter()
from Sound.Effects import echo # in code: echo.echofilter()
reload(echo) # reload imported module
from Sound.Effects.echo import * # in code: echofilter()
import sys # system specific parameters and functions
sys.exit("bye") # exit the program (raises SystemExit)
sys.path.append('c:\\') # module search path, init from PYTHONPATH
import math # math related operations
print math.sqrt(9) # print 3
import random # generate pseudo-random numbers
f = random.random() # random float in range [0.0,1.0)
i = randint(1,10) # random int in range [1,10]
print random.choice(['a','b']) # returns a random member of the sequence
from datetime import date # date and calendar related functions
a = date.today()-date(1981,11,21) # supports date arithmetics
print age.days # how old am i in days?
import zlib # compression related functions
c = zlib.compress("hello world") # c is a byte string (shorter hopefully)
print zlib.decompress(c) # prints "hello world"
# command execution in runtime
exec('x=1') # interpret new python code in runtime
y = eval('x+1') # evaluate an python expression and ret it
import os # operating system interface
os.system('del *.txt') # run commands in the shell (blocking)
os.startfile("calc.exe") # run commands in the shell (nonblocking)
stdin,stdouterr = os.popen4('dir /b') # returns file handles for child comm.
print stdouterr.read() # print the output, popen3 gives 3 streams
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