Python

1. print 1 / 3, 5 / 2, -7 / 3        # print out float numbers with at most 15 digits;
   print 1 // 3, 5 // 2, -7 // 3    # print out integers => 0 , 2,  -2;
   print "Let's talk about %s" % my_name     # format print, variable my_name = 'Wade';
   print "." * 10 # print out 10 .s
   fat_cat = """                    # documentation comments """
   I'll do a list:
   \t* cat food
   \t* Fishes
   \t* Catnip\n\t* Grass
   """　　# you also can use '''
   
   from sys import argv
   script, first, second, third = argv
   
   a = 'Hello'
   a[1:3]        >>> 'el'
   my_string[::-1] # reverse a string
   
   for string and list, these are common builtin function:
   1.a = [1, 2, 3]　　a.append(5); del a; a.pop(2); 2 in a; for num in a: ...
   
   a = [4, 2, 1, 5]
   sorted(a)    >>>[1, 2, 4, 5]
   b = ':'.join(a)   >>> string func not working here
   b.split(':')    >>> 
   
   range(3)   >>> [0, 1, 2]

2. Ctrl-D to get out of python in the terminal.
3. put these characters in the first line to use UTF-8 coding: #--coding: utf-8--
4. a little debugging technique: checking code from back to front word by word.
5. function round(): round(1.733) #=> 2.0
6. %s using str() to do convertion, and %r using repr(); %r is used for debugging, since it displays the "raw" data of the variable.
7. if else elif; raw_input('-->')
8. If you have somethin to ask, just type: pydoc raw_input; q to quit
9. in python interpretor, you can use dir() or help() function to get help.
10. string, tuple is fixed, and list can shrink and grow.
11. (x, y) = (1, 2)  #parallel assignment
12. A Python module can be run directly or it can be imported and used by some other module. When a Python file is run directly, the special variable "__name__" is set to "__main__". Therefore, it's common to have the boilerplate if __name__ ==... shown above to call a main() function when the module is run directly, but not when the module is imported by some other module.
13. open(name[, mode[, buffer]]): mode: "r": reading, "w" writing, "a": appending; from_file.close()
14. let f be a file, f.seek(0) : moving to the start of the file. Each time you do f.readline() you're reading a line from the file, and moving the read head to right after the \n that ends that file.
15. when you do import, you do not need to put the .py at the end to import it. you have file ex07.py, you should do: import ex07. In terminal, you type pydoc somemodule.somefunction, then it will come up some documents describing it.
    
16. pop(0), pop(-1)
17. exit(0): exit program
18. every if-statement must have an else, if not you have to use a die function and print out a error message. nest if less than 2 times! or put it in another function.
19. never use while loop unless you want to loop forever. Use print to debug. Code a little, run a little, fix a little.

20. foo = ['a', 'b', 'c', 'd', 'e']
    from random import choice
    print choice(foo)

21. collections.OrderedDict: http://pymotw.com/2/collections/ordereddict.html collections.OrderedDict remembers the order when things added.
22. pass, break, continue: http://www.tutorialspoint.com/python/python_loop_control.htm
23. thing = MyClass() # instantiate a class
24. .strip() removes all whitespace at the start and end, including spaces, tabs, newlines and carriage returns.
25. list.count(obj) # return times obj occurs in list.

26. shuffleMe = [0,1,2,3,4,5,6,7,8,9]
    random.shuffle(shuffleMe)
    print(shuffleMe)
    //[2, 4, 6, 3, 7, 8, 0, 5, 9, 1]
    
    

    pickChars ="some things to pick"
    print random.sample(pickChars,6)    //['c', 's', 'e', 'n', 'g', ' ']
    pickNums =[0,1,2,3,4,5,6,7,8,9]
    print random.sample(pickNums,5)     //[6, 4, 5, 9, 8]
    
    

    >>> phrase ='lexical semantics'
    >>> phrase.capitalize()    //'Lexical semantics'
    >>> phrase.upper()         //'LEXICAL SEMANTICS'
    >>> phrase.title()         //'Lexical Semantics'
    
    b = 'This is a string'
    b[k] = b[-len(b) + k]
    a = '0123456789'
    c = a[start:stop:step]
    a[2:6] = a[2:6:1] = '2345'
    a[:] = a[0:len(a):1]
    a[::2] = '02468'
    a[6:2:-1] = '6543'
    a[::-1] = a[-1::-1]    #reverse a string

27.  list2 = list1[:]  # copying lists using slice
28. The method replace() returns a copy of the string in which the occurrences of old have been replaced with new, optionally restricting the number of replacements to max.  str.replace(old, new[, max])

29. a,b,_ = 0,1,2  -> dismisses the 2
    a,_,c = 0,1,2  -> dismisses the 1
    _,_,c = 0,1,2  -> dismisses the 0 and 1
    a,b = 0,1,2    -> ERROR: too many values to unpack
    t = 0,1,2      -> creates the tuple t=(0,1,2), 'packing' the values
    v, w, x = t    -> sets v=0, w=1 and x=2, 'unpacking' the values
    v, w = t       -> ERROR: need more than 2 values to unpack
    
    >>> range(10)
    [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
    >>> range(5, 10)

    [5, 6, 7, 8, 9]
    >>> range(0, 10, 3)
    [0, 3, 6, 9]
    >>> range(-10, -100, -30)
    [-10, -40, -70]
    
    

    >>> a = ['Mary', 'had', 'a', 'little', 'lamb']
    >>> for i in range(len(a)):
    　　     print i, a[i]

30. Most of the uses of inheritance can be simplified or replaced with composition, and multiple-inheritance should be avoided at all costs.
31. parent and child interact in 3 ways: 1. imply(call parent method); 2. override; 3. overload.
32. When invoking the Python command line interpreter with the -t option, it issues warnings about code that illegally mixes tabs and spaces. When using -tt these warnings become errors. These options are highly recommended!
33. using \x, \u or \U escapes is the preferred way to include non-ASCII data in string literals.
34. exit(0): successful exit; exit(1):there was some issue / error / problem and that is why the program is exiting.

35. >>> range(10)
    [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

36. "\" or "()" to continue multiple lines: 

    >>> b = ((i1 < 20) and
    ...      (i2 < 30) and
    ...      (i3 < 40))
    >>> b
    True
    >>>
    >>> b = (i1 < 20) and \
    ...     (i2 < 30) and \
    ...     (i3 < 40)
    >>> 
    >>> b
    True

37. from datetime import datetime
    now = datetime.now()
    print now # 2013-04-16 20:38:35.308367
    # now.day now.year now.month represents day year month
    # some_string.isalpha() return True or False to test if the variable contains only characters

38. # splat arguments:
    def favorite_actors(*args):
        """Prints out your favorite actorS (plural!)"""
        print "Your favorite actors are:" , args
    #  Splat arguments are an arbitrary number of arguments, and you use them by preceding your parameter with a *.

39. from module import function    # function import : from math import sqrt;  sqrt();
    import module                  # generic import  : math.sqrt()
    3 from math import *             # import all thins in math; not recommended

40. Built in function:
    1. max(): takes any number of arguments and returns the largest one.
    2. min(): opposite of max();
    3. abs(): returns the absolute value of the number it takes as an argument
    4. type(): it returns the type of the data it receives as an argument.
41. Dictionary: 
    1. d = ['key1':1, 'key2':2, 'key3':3]
    2. d['key2'] = 5
    3. d[new_key] = new_value    # add new pair
    4. del d[key_name]                 # remove item based on key_name
    5. d.remove(value)                 # will remove the the first item from d that has a value equal to value

42. 1 #!/usr/bin/python
    2 
    3 aList = [123, 'xyz', 'zara', 'abc', 'xyz'];
    4 
    5 aList.remove('xyz');   // remove the firs occurrence of a value from a list.
    6 print "List : ", aList;
    7 aList.remove('abc');
    8 print "List : ", aList;

    n = [1, 3, 5]
    n.pop(1)  // .pop() method returns the last element, and remove it from the list.
    # Returns 3 (the item at index 1)
    print n
    # prints [1, 5]
    
    range(arg1,[arg2, [arg3]]):
    one arg:
        range(1) => [0];          range(2) => [0, 1]
    two arg: 
        range(1, 3) => [1, 2]
    three arg: 
    　　range(2, 8, 3) #=> [2, 5];       range(2, 9, 3)  #=> [2, 5, 8]

43. li = ['server=mpilgrim', 'uid=sa', 'database=master', 'pwd=secret']
    s = ";".join(li)
    # s => 'server=mpilgrim;uid=sa;database=master;pwd=secret'
    s.split(";") 
    # ['server=mpilgrim', 'uid=sa', 'database=master', 'pwd=secret']
    s.split(";", 1)
    # ['server=mpilgrim', 'uid=sa;database=master;pwd=secret']

44. random.randint(start,stop): return start less or equal than N, N bigger or equal than stop.
45. raw_input(">>>"): returns a string of user input
46. while/else is similar to if/else, but there is a difference: the else block will execute anytime the loop condition is evaluated to False. This means that it will execute if the loop is never entered or if the loop exits normally. If the loop exits as the result of a break, the else will not be executed.
47. A new built-in function, enumerate(), will make certain loops a bit clearer. enumerate(thing), where thing is either an iterator or a sequence, returns a iterator that will return a tuple (0, thing[0]), (1, thing[1]), (2, thing[2]), and so forth.
48. [pair for pari in enumerate(mylist)] 

    x = [1, 5, 7, 3, 8]
    print("x = "+str(x))
    for ind, itm in enumerate(x):
        print("x["+str(ind)+"] = "+str(itm))
    '''The ouput of this snippet is:
    x = [1, 5, 7, 3, 8]
    x[0] = 1
    x[1] = 5
    x[2] = 7
    x[3] = 3
    x[4] = 8'''

49. alist = ['a1', 'a2', 'a3']
    blist = ['b1', 'b2', 'b3']
    
    for a, b in zip(alist, blist):
        print a, b
    
    alist = ['a1', 'a2', 'a3']
    blist = ['b1', 'b2', 'b3']
    
    for i, (a, b) in enumerate(zip(alist, blist)):
        print i, a, b

    Results:
    0 a1 b1
    1 a2 b2
    2 a3 b3

50. for/else:  else statement is executed after the for, but only if the for ends normally—that is, not with a break
51. the Python function items() will iterate over a dictionary and return those key/value pairs. items() returns an array of tuples* with each tuple consisting of a key/value pair from the dictionary Note that the items() function doesn't return key/value pairs in any specific order.

    d = {
        "Name": "Guido",
        "Age": 56,
        "BDFL": True
    }
    d.items()    // d.clear()  #=> d = {}
    # => [('BDFL', True), ('Age', 56), ('Name', 'Guido')]

52. The keys() function returns an array of the dictionary's keys, and; The values() function returns an array of the dictionary's values.
53. list comprehensions: 

    doubles_by_3 = [x*2 for x in range(1,6) if (x*2)%3 == 0]
    # => [6]

54. list slicing: [start: end: stride]
55. lambda construct: (anonymous function) you can put a lambda definition anywhere a function is expected. if we need it only once and it's quite simple

    >>> def f (x): return x**2
    ... 
    >>> print f(8)
    64
    >>> 
    >>> g = lambda x: x**2
    >>> 
    >>> print g(8)
    64
    10 foo = [2, 18, 9, 22, 17, 24, 8, 12, 27]
    11 print filter(lambda x: x % 3 == 0, foo)  # [18, 9, 24, 12, 27] 
    12 print reduce(lambda x, y: x + y, foo)    # sum: 139
    13 print map(lambda x: x * 2 + 10, foo)     #[14, 46, 28, 54, 44, 58, 26, 34, 64]
    # lambda syntax:  lambda variable: expression

56. filter(): filter() calls our lambda function for each element of the list, and returns a new list that contains only those elements for which the function returned "True".
57. bin() function: bin takes an integer as input and returns the binary representation of that integer in a string. You can also represent numbers in base 8 and base 16 using the oct() and hex()functions.
58. class: variables that are available everywhere (global variable); variables that are only available to members of a certain class (member variables); variables that are only available to particular instances of a class (instance variables)
59. overide: 

    class DerivedClass(Base):
       def some_method(self):
           super(DerivedClass, self).meth()
    # meth() is a method of superclass 调父类方法

60. member variable should be accessed by self.var1;
61. __repr__():
62. 
63. IO:  my_file = open("output.txt", "r+")  #  r+: read and write; w: only write; r: only read; a: append new data;  file.readline()
64. file objects contain a special pair of built-in methods:__enter__() and __exit__(). The details aren't important, but what isimportant is that when a file object's__exit__() method is invoked, it automatically closes the file. How do we invoke this method? With with and as：

    with open("text.txt", "w") as textfile:
        textfile.write("Success!")

65. Python file objects have a closed attribute which is True when the file is closed and False otherwise.
66. function: python function will return a value or None instead if no return statement.
67. Everything between the triple quotes is the function's doc string, which documents what the function does. doc string, if exists, must be the first thing defined in a function. Function is an object, and it has attributes. 

    # example code: in module(d01.py)
    def ex(params):
        ''' blablabla '''
        
    /// console:
    >>>import d01
    >>>print ex.__doc__        #=> blablabla

68. once you import a module, you can reference any of its public functions classes or attributes. You can access its function with module.function
69. Python looks in several places if you try to import a module. Specifically, it looks in all the directories defined in sys.path(this just is a list)

    >>> import sys
    >>> sys.path
    [blabla...]
    >>>sys
    <module 'sys' (built-in)>          // the sys module is written in C
    >>>sys.path.append('/my/new/path') // sys.path is a list of directories names that constitute the current search path

70. Everything is an object. Meaning that it can be assigned to a variable or passed as an argument to a function.
71. Modules are objects, and all modules have a built-in attribute __name__, If you import the module, then __name__ is the module's filename, without a directory path or file extension. But you can also run the module directly as a standalone program, in which case __name__ will be a special default value, __main__.
72. Lists:

    >>> li
    ['a', 'b', 'mpilgrim', 'z', 'example']
    >>> li.append("new")
     3 >>> li.index('mpilgrim') #=> 3  .index('value') method returns the first occurrence of a value's index.
     3 >>> 'c' in li    #=> False      use in to test weather an item is in the list
    >>> li
    ['a', 'b', 'mpilgrim', 'z', 'example', 'new']
    >>> li.insert(2, "new")
    >>> li
    ['a', 'b', 'new', 'mpilgrim', 'z', 'example', 'new']
    >>> li.extend(["two", "elements"])     # concatenates two lists.
    >>> li
    ['a', 'b', 'new', 'mpilgrim', 'z', 'example', 'new', 'two', 'elements']
    12 >>> li = ['a','b','mpilgrim']
    13 >>> li = li + ['c','d']
    14 ['a','b','mpilgrim','c','d']
    15 //+ returns a new concatenated list but .extend() method alters the original list(faster); li += ['c','d'] works as extend.
    16 >>> li = [1, 2] * 3
    17 [1, 2, 1, 2, 1, 2] // * repeater. 
    18 // tuple: t = ("a", "b", "mpilgrim", "z", "example")
    19 // tuple has no method, but can do this: 'z' in t >>> True
    20 // why use tuple:1. faster than list(循环最好用tuple); 2.tuples can be used as dictionary keys(list can't, dictionary keys must be immutable) 
    

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