Python Quick Start (4) sorting, dictionary, and files

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Python Sorting:
1. The simplest sorting method:

#sorted(list) function: which takes a list and returns a new list with those elements in sorted ordera = [5, 1, 4, 3]print sorted(a)  ## [1, 3, 4, 5]print a  ## [5, 1, 4, 3]#The sorted() function can be customized though optional arguments. The sorted() optional argument reverse=True, e.g. sorted(list, reverse=True), makes it sort backwards.strs = ['aa', 'BB', 'zz', 'CC']print sorted(strs)  ## ['BB', 'CC', 'aa', 'zz'] (case sensitive)print sorted(strs, reverse=True)   ## ['zz', 'aa', 'CC', 'BB']

2. Use key = func for custom sorting
# 1.For example with a list of strings, specifying key = len (the built in len () function) sorts the strings by length, from shortest to longest. the sort calllen () for each string to get the list of proxy length values, and the sorts with those proxy values.

strs = ['ccc', 'aaaa', 'd', 'bb']print sorted(strs, key=len)  ## ['d', 'bb', 'ccc', 'aaaa']#2.As another example, specifying "str.lower" as the key function is a way to force the sorting to treat uppercase and lowercase the same:## "key" argument specifying str.lower function to use for sortingprint sorted(strs, key=str.lower)  ## ['aa', 'BB', 'CC', 'zz']#3.You can also pass in your own MyFn as the key function, like this:## Say we have a list of strings we want to sort by the last letter of the string.    strs = ['xc', 'zb', 'yd' ,'wa']    ## Write a little function that takes a string, and returns its last letter.    ## This will be the key function (takes in 1 value, returns 1 value).    def MyFn(s):  return s[-1]    ## Now pass key=MyFn to sorted() to sort by the last letter:    print sorted(strs, key=MyFn)  ## ['wa', 'zb', 'xc', 'yd']

3. sort () method

#the sort() method on a list sorts that list into ascending orderlist.sort()#the sort() method changes the underlying list and returns None

Tuple
Tuple is unchangeable and the size cannot be changed.
# Tuples play a sort of "struct" role in Python -- a convenient way to pass around a little logical, fixed size bundle of values.

  tuple = (1, 2, 'hi')  print len(tuple)  ## 3  print tuple[2]    ## hi  tuple[2] = 'bye'  ## NO, tuples cannot be changed  tuple = (1, 2, 'bye')  ## this works, it created a new tuple

List advanced:
1.

  nums = [1, 2, 3, 4]  squares = [ n * n for n in nums ]   ## [1, 4, 9, 16]  [ expr for var in nums] -- ## the expr to its left is evaluated once for each element to give the values for the new list.

2.

  strs = ['hello', 'and', 'goodbye']  shouting = [ s.upper() + '!!!' for s in strs ]  ## ['HELLO!!!', 'AND!!!', 'GOODBYE!!!']

3. You can also add the if judgment statement:

  ## Select values <= 2  nums = [2, 8, 1, 6]  small = [ n for n in nums if n <= 2 ]  ## [2, 1]  ## Select fruits containing 'a', change to upper case  fruits = ['apple', 'cherry', 'bannana', 'lemon']  afruits = [ s.upper() for s in fruits if 'a' in s ]  ## ['APPLE', 'BANNANA']

Dictionary dict :{}
Ing type/hash table

# The contents of a dict can be written as a series of key: value pairs within braces {}, e.g. dict = {key1: value1, key2: value2 ,...}. the "empty dict" is just an empty pair of curly braces {}.

The dictionary is the only ing type (hash table) in python. Common member methods:
Keys ()
Values ()
Items ()

Dic = {"name": "well", "age": 20, "gender": "male"} dic. keys () # Return All keydic ['name'] # Return "well"

  ## Can build up a dict by starting with the the empty dict {}  ## and storing key/value pairs into the dict like this:  ## dict[key] = value-for-that-key  dict = {}  dict['a'] = 'alpha'  dict['g'] = 'gamma'  dict['o'] = 'omega'  print dict  ## {'a': 'alpha', 'o': 'omega', 'g': 'gamma'}  print dict['a']     ## Simple lookup, returns 'alpha'  dict['a'] = 6       ## Put new key/value into dict  'a' in dict         ## True  ## print dict['z']                  ## Throws KeyError  if 'z' in dict: print dict['z']     ## Avoid KeyError  print dict.get('z')  ## None (instead of KeyError)      ## By default, iterating over a dict iterates over its keys.  ## Note that the keys are in a random order.  for key in dict: print key  ## prints a g o    ## Exactly the same as above  for key in dict.keys(): print key  ## Get the .keys() list:  print dict.keys()  ## ['a', 'o', 'g']  ## Likewise, there's a .values() list of values  print dict.values()  ## ['alpha', 'omega', 'gamma']  ## Common case -- loop over the keys in sorted order,  ## accessing each key/value  for key in sorted(dict.keys()):print key, dict[key]    ## .items() is the dict expressed as (key, value) tuples  print dict.items()  ##  [('a', 'alpha'), ('o', 'omega'), ('g', 'gamma')]  ## This loop syntax accesses the whole dict by looping  ## over the .items() tuple list, accessing one (key, value)  ## pair on each iteration.  for k, v in dict.items(): print k, '>', v  ## a > alpha    o > omega     g > gamma

Dictionary and format:
# The % operator works conveniently to substitute values from a dict into a string by name:

  hash = {}  hash['word'] = 'garfield'  hash['count'] = 42  s = 'I want %(count)d copies of %(word)s' % hash  # %d for int, %s for string  print s  # 'I want 42 copies of garfield'

Del:
# The "del" operator does deletions. in the simplest case, it can remove the definition of a variable, as if that variable had not been defined. del can also be used on list elements or slices to delete that part of the list and to delete entries from a dictionary.

  var = 6  del var  # var no more!    list = ['a', 'b', 'c', 'd']  del list[0]     ## Delete first element  del list[-2:]   ## Delete last two elements  print list      ## ['b']  dict = {'a':1, 'b':2, 'c':3}  del dict['b']   ## Delete 'b' entry  print dict      ## {'a':1, 'c':3}

File:

  # Echo the contents of a file  f = open('foo.txt', 'rU')  ## The special mode 'rU' is the "Universal" option for text files where it's smart about converting different line-endings so they always come through as a simple '\n'.   ## Instead of 'r', use 'w' for writing, and 'a' for append.  for line in f:   ## iterates over the lines of the fileprint line,    ## trailing, so print does not add an end-of-line char   ## since 'line' already includes the end-of line.  f.close()

# The f. readlines () method reads the whole file into memory and returns its contents as a list of its lines.
# The f. read () method reads the whole file into a single string, which can be a handy way to deal with the text all at once, such as with regular expressions we'll see later.
# For writing, f. write (string) method is the easiest way to write data to an open output file.

To read unicode files, you can use the codecs module:

    import codecsf = codecs.open('foo.txt', 'rU', 'utf-8')for line in f:  # here line is a *unicode* string

Ex. The number of words in the statistical file:
Display format:

Word1 count1
Word2 count2
......

def word_count_dict(filename):  """Returns a word/count dict for this filename."""  # Utility used by count() and Topcount().  word_count = {}  # Map each word to its count  input_file = open(filename, 'r')  for line in input_file:words = line.split()for word in words:  word = word.lower()  # Special case if we're seeing this word for the first time.  if not word in word_count:word_count[word] = 1  else:word_count[word] = word_count[word] + 1  input_file.close()  # Not strictly required, but good form.  return word_count