http://docs.python.org/tutorial/introduction.html
Print will always add a newline char, unless you put a comma after it so in a loop, print a will print a on each line, while print a, will have no new lines
+ contac strings, or add numbers = (assignment) == (equal to) > < >= <=
float(a) #converts a to float int(a) #converts a to integer long(a) #converts a to long str(a) #converts a to string
'doesn\'t' "doesn't" 'He said "Yes"'
hello = r"This is a rather long string containing\n\ several lines of text much as you would do in C."
word = 'HI ' + 'There' word*5 #prints Hi There five times
word = HelpA word[4] #A word[0:2] #He word[2:4] #lp word[:2] #He word[2:] #lpA word[-1] #A word[-2] #p word[-2:] #pA word[:-2] #Hel word[-100:] #HelpA word[-10] # error word[0] = x # ERROR, words cannot be changed ('x' + word[1:] is good)
Each element is its own type, string int double...
a = ['spam', 'eggs', 100, 1234] >>> a[0] 'spam' >>> a[3] 1234 >>> a[-2] 100 >>> a[1:-1] ['eggs', 100] >>> a[:2] + ['bacon', 2*2] ['spam', 'eggs', 'bacon', 4] >>> 3*a[:3] + ['Boo!'] ['spam', 'eggs', 100, 'spam', 'eggs', 100, 'spam', 'eggs', 100, 'Boo!']
>>> a[0] 'spam' >>> a[3] 1234 >>> a[-2] 100 >>> a[1:-1] ['eggs', 100] >>> a[:2] + ['bacon', 2*2] ['spam', 'eggs', 'bacon', 4] >>> 3*a[:3] + ['Boo!'] ['spam', 'eggs', 100, 'spam', 'eggs', 100, 'spam', 'eggs', 100, 'Boo!']
>>> # Replace some items: ... a[0:2] = [1, 12] >>> a [1, 12, 123, 1234] >>> # Remove some: ... a[0:2] = [] >>> a [123, 1234] >>> # Insert some: ... a[1:1] = ['bletch', 'xyzzy'] >>> a [123, 'bletch', 'xyzzy', 1234] >>> # Insert (a copy of) itself at the beginning >>> a[:0] = a >>> a [123, 'bletch', 'xyzzy', 1234, 123, 'bletch', 'xyzzy', 1234] >>> # Clear the list: replace all items with an empty list >>> a[:] = [] >>> a []
>>> a = ['a', 'b', 'c', 'd'] >>> len(a) 4
>>> q = [2, 3] >>> p = [1, q, 4] >>> len(p) 3 >>> p[1] [2, 3] >>> p[1][0] 2 >>> p[1].append('xtra') # See section 5.1 >>> p [1, [2, 3, 'xtra'], 4] >>> q [2, 3, 'xtra']
>>> # Fibonacci series: ... # the sum of two elements defines the next ... a, b = 0, 1 >>> while b < 10: ... print b ... a, b = b, a+b ... 1 1 2 3 5 8
>>> x = int(raw_input("Please enter an integer: ")) Please enter an integer: 42 >>> if x < 0: ... x = 0 ... print 'Negative changed to zero' ... elif x == 0: ... print 'Zero' ... elif x == 1: ... print 'Single' ... else: ... print 'More' ... More
>>> # Measure some strings: ... a = ['cat', 'window', 'defenestrate'] >>> for x in a: ... print x, len(x) ... cat 3 window 6 defenestrate 12
To modify the list
>>> for x in a[:]: # make a slice copy of the entire list ... if len(x) > 6: a.insert(0, x) ... >>> a ['defenestrate', 'cat', 'window', 'defenestrate']
>>> range(10) [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] >>> range(5, 10) [5, 6, 7, 8, 9]
>>> a = ['Mary', 'had', 'a', 'little', 'lamb'] >>> for i in range(len(a)): ... print i, a[i] ... 0 Mary 1 had 2 a 3 little 4 lamb
More loops here http://docs.python.org/tutorial/datastructures.html#tut-loopidioms
The break statement, like in C, breaks out of the smallest enclosing for or while loop. The continue statement, also borrowed from C, continues with the next iteration of the loop.
>>> for n in range(2, 10): ... for x in range(2, n): ... if n % x == 0: ... print n, 'equals', x, '*', n/x ... break ... else: ... # loop fell through without finding a factor ... print n, 'is a prime number' ... 2 is a prime number 3 is a prime number 4 equals 2 * 2 5 is a prime number 6 equals 2 * 3 7 is a prime number 8 equals 2 * 4 9 equals 3 * 3
The pass statement does nothing. It can be used when a statement is required syntactically but the program requires no action. For example:
>>> while True: ... pass # Busy-wait for keyboard interrupt (Ctrl+C) ... >>> class MyEmptyClass: ... pass ...
The first statement of the function body can optionally be a string literal; this string literal is the function's documentation string, or docstring
>>> def fib(n): # write Fibonacci series up to n ... """Print a Fibonacci series up to n.""" ... a, b = 0, 1 ... while a < n: ... print a, ... a, b = b, a+b ... >>> # Now call the function we just defined: ... fib(2000) 0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597
since fib does not return anything, you can assign the actual function to another variable \ kinda like a function alias
>>> fib <function fib at 10042ed0> >>> f = fib >>> f(100) 0 1 1 2 3 5 8 13 21 34 55 89
functions without return, actually return 'None'
>>> fib(0) >>> print fib(0) None
>>> def fib2(n): # return Fibonacci series up to n ... """Return a list containing the Fibonacci series up to n.""" ... result = [] ... a, b = 0, 1 ... while a < n: ... result.append(a) # see below ... a, b = b, a+b ... return result ... >>> f100 = fib2(100) # call it >>> f100 # write the result [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]
def ask_ok(prompt, retries=4, complaint='Yes or no, please!'): while True: ok = raw_input(prompt) if ok in ('y', 'ye', 'yes'): return True if ok in ('n', 'no', 'nop', 'nope'): return False retries = retries - 1 if retries < 0: raise IOError('refusenik user') print complaint
The default values are evaluated at the point of function definition in the defining scope, so that
i = 5 def f(arg=i): print arg i = 6 f() # will print 5
Important Warning The default value is evaluated only once. This makes a difference when the default is a mutable object such as a list, dictionary, or instances of most classes. For example, the following function accumulates the arguments passed to it on subsequent calls:
def f(a, L=[]): L.append(a) return L print f(1) # [1] print f(2) # [1, 2] print f(3) # [1, 2, 3] IF you don't want this behavior then: def f(a, L=None): if L is None: L = [] L.append(a) return L
You don't have to call parameters in order, if you reference it's keyword
def parrot(voltage, state='a stiff', action='voom', type='Norwegian Blue'): print "-- This parrot wouldn't", action, print "if you put", voltage, "volts through it." print "-- Lovely plumage, the", type print "-- It's", state, "!" parrot(1000) parrot(action = 'VOOOOOM', voltage = 1000000) parrot('a thousand', state = 'pushing up the daisies') parrot('a million', 'bereft of life', 'jump')
When a final formal parameter of the form name is present, it receives a dictionary (see Mapping Types — dict) containing all keyword arguments except for those corresponding to a formal parameter. This may be combined with a formal parameter of the form *name (described in the next subsection) which receives a tuple containing the positional arguments beyond the formal parameter list. (*name must occur before name.) For example, if we define a function like this:
def cheeseshop(kind, *arguments, **keywords): print "-- Do you have any", kind, "?" print "-- I'm sorry, we're all out of", kind for arg in arguments: print arg print "-" * 40 keys = keywords.keys() keys.sort() for kw in keys: print kw, ":", keywords[kw] cheeseshop("Limburger", "It's very runny, sir.", "It's really very, VERY runny, sir.", shopkeeper='Michael Palin', client="John Cleese", sketch="Cheese Shop Sketch") #WOULD PRINT OUT -- Do you have any Limburger ? -- I'm sorry, we're all out of Limburger It's very runny, sir. It's really very, VERY runny, sir. ---------------------------------------- client : John Cleese shopkeeper : Michael Palin sketch : Cheese Shop Sketch
Finally, the least frequently used option is to specify that a function can be called with an arbitrary number of arguments. These arguments will be wrapped up in a tuple (see Tuples and Sequences). Before the variable number of arguments, zero or more normal arguments may occur.
def write_multiple_items(file, separator, *args): file.write(separator.join(args))
