LectureNotesOnPython.rst: added Conditional Expression, Generator, any and all, sets, Counter, defaultdict, keyword arguments.

1 files changed, 195 insertions, 0 deletions

diff --git a/LectureNotesOnPython.rst b/LectureNotesOnPython.rst
index c8d4c06..eebb243 100644
--- a/LectureNotesOnPython.rst
+++ b/LectureNotesOnPython.rst
@@ -2120,7 +2120,202 @@ school.py
 
 omg
 
+
+其它有用的Python语言特性
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+
+条件表达
+```````````````````````````````````````````````````
+
+把 if else 写在一行。看函数 take_log3。
+
+
+.. code:: python
+
+         import math
+         
+         def take_log(x):
+             if x > 0:
+                 return math.log(x)
+             else:
+                 raise Exception('The input is not a positive number.')
+             
+         def take_log2(x):
+             if x > 0:
+                 return math.log(x)
+             else:
+                 return float('nan')
+             
+         def take_log3(x):
+             return math.log(x) if x > 0 else float('nan')
+         
+         
+         
+         x = -1
+         try:
+             result = take_log(x)
+             print('result from take_log is %g.' % (result))
+         except:
+             print('x is not a good value. ')
+             
+         
+         result = take_log2(x)
+         print('result from take_log2 is %g.' % (result))
+         result = take_log3(x)
+         print('result from take_log3 is %g.' % (result))
+         
+
+
+Generator表达
+```````````````````````````````````````````````````
+
+Generator也是一个对象，对象里面有 __iter__ 与 __next__ 方法。 当所有的元素都遍历完之后，抛出 **StopIteration** 异常。
+
+.. code:: python
+	  
+         g = (x**2 for x in range(3))
+         
+         print(next(g))
+         print(next(g))
+         print(next(g))
+         print(next(g))
+
+
+可以用 for loop 遍历 g 中的所有元素。
+
+.. code:: python
 	  
+         g = (x**2 for x in range(3))
+         
+         for x in g:
+             print(x)
+
+
+
+any 与 all
+```````````````````````````````````````````````````
+
+any/all 是个函数，可以接收 list 或 generator 类型的参数。
+
+
+.. code:: python
+
+
+         any([False, False, True]) # True
+         
+         developed_countries = ['canada', 'germany', 'japan', 'uk', 'swiss', 'norway']
+         
+         f = open('brexit-news.txt')
+         news = f.read()
+         f.close()
+         print( any(word.strip().lower() in developed_countries for word in news.split()) )
+         print( all(word.strip().lower() in developed_countries for word in news.split()) )
+         
+
+
+集合 Sets
+```````````````````````````````````````````````````
+
+.. code:: python
+	  
+         set('123')
+         set([1,2,3])
+         
+         def all_letters_unique(word):
+             return len(set(word)) == len(word)
+         
+         def all_letters_vowel(word):
+             return set(word) <= set('aeiou')
+         
+         def compare_vocaburary(a, b):
+             return set(a) > set(b)
+         
+         def vocaburary_diff(a, b):
+             return list(set(a) - set(b))
+         
+         def vocaburary_common(a, b):
+             return list(set(a).intersection(set(b)))
+         
+         print( all_letters_unique('unique') )
+         print( all_letters_vowel('ou') )
+         print( compare_vocaburary(['he', 'knows', 'big', 'small'], ['he', 'knows', 'small']) )
+         print( vocaburary_diff(['he', 'knows', 'big', 'small'], ['he', 'knows', 'small']) )
+         print( vocaburary_common(['he', 'knows', 'big', 'small'], ['he', 'knows', 'small']) )
+         
+
+
+计数 Counter
+```````````````````````````````````````````````````
+
+与字典有点像。
+
+.. code:: python
+
+
+         from collections import Counter
+         
+         c = Counter('donald') # Counter({'d': 2, 'o': 1, 'n': 1, 'a': 1, 'l': 1})
+         c['d'] # 2
+         c['x'] # 0, not a KeyError
+         c.most_common() # [('d', 2), ('o', 1), ('n', 1), ('a', 1), ('l', 1)]
+         
+         vote = Counter(['yes', 'no', 'yes', 'yes', 'no']) # Counter({'yes': 3, 'no': 2})
+
+
+
+defaultdict
+```````````````````````````````````````````````````
+
+可以避免第一次加 value 时 key 不在引起的 KeyError。
+
+.. code:: python
+	  
+         from collections import defaultdict
+         
+         def seperate_odd_and_even(lst):
+             d = defaultdict(list) # the factory is list
+             for x in lst:
+                 if x % 2 != 0:
+                     d['odd'].append(x)
+                 else:
+                     d['even'].append(x)
+             return d
+                
+         def unique_odd_and_even(lst):
+             d = defaultdict(set) # the factory is set
+             for x in lst:
+                 if x % 2 != 0:
+                     d['odd'].add(x)
+                 else:
+                     d['even'].add(x)
+             return d
+         
+                 
+         result = seperate_odd_and_even([1,2,3,4,5])
+         result['odd']
+         result['even']
+         
+         result2 = unique_odd_and_even([1,1,2,2,3,3])
+         result2['odd']
+         result2['even']
+
+
+收集keyword args
+```````````````````````````````````````````````````
+
+.. code:: python
+	  
+         def printall(*args, **kwargs):
+             print(args)   # args is a tuple
+             print(kwargs) # kwargs is a dictionary
+             
+             
+         printall(1,2,3, a=1,b=2,c=3)
+
+
+
+
 参考
 ------