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python_fragment_NB.txt
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1. In Python, max with a key argument does 'argmax'
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2. re.findall(r'\w+', text.lower())
查找文件中所有单词，返回list
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3. python中的三个读read(),readline()和readlines()
read()：每次读取整个文件，它通常用于将文件内容放到一个字符串变量中。
readline()：每次只读取一行
readlines()：自动将文件内容分析成一个行的列表
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4. from collections import Counter
Counter：字典的子类，用于统计哈希对象。计数器（Counter）是一个容器，用来跟踪值出现了多少次。构造函数可以调用序列，包含key和计数的字典，或使用关键字参数。如：
printcollections.Counter(['a', 'b', 'c', 'a', 'b', 'b'])
printcollections.Counter({'a':2, 'b':3, 'c':1})
printcollections.Counter(a=2, b=3, c=1)
counter.most_common(n)，返回一个TopN列表
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5. max(arg1, arg2, *args[, key])
本函数是迭代对象iterable进行比较，找出最大值返回。当key参数不为空时，就以key的函数对象为判断的标准。
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6.
"String".upper():
Methods that use dot notation only work with strings.
len() and str() can work on other data types
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7. 获取时间
7.1 获取当前时间
from datetime import datetime
now = datetime.now()
当前年： now.year
当前月： now.month
当前天： now.day
当前时： now.hour
当前分： now.miinute
当前秒： now.second
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8. not, and, or的顺序
not is evaluated first;
and is evaluated next;
or is evaluated last.
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9.
9.1 Python isalpha() 方法检测字符串是否只由字母组成
str.isalpha()
9.2 isdigit()方法检查字符串是否只包含数字(全由数字组成)
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10.python参数的知识
10.1 位置参数
在参数名之前使用一个星号，就是让函数接受任意多的位置参数。
def multiply(*args)
python把参数收集到一个元组中，作为变量args
10.2 关键字参数
python在参数名之前使用2个星号来支持任意多的关键字参数
10.3 混合参数类型
任意的位置参数和关键字参数可以和其他标准的参数声明一起使用。 python中他们的次序是重要的。参数归为4类，不是所有的类别都需要。他们必须按下面的次序定义，不用的可以跳过。
1）必须的参数
2）可选的参数
3）过量的位置参数
4）过量的关键字参数
def complex_function(a, b=None, *c, **d):
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11. __doc__,__file__
documentation string： print __doc__
the path of our module： __file__
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12. functions
def f():
"""This function prints a message """
print "Today it is a cloudy day"
print isinstance(f, object)
print id(f)
print f.func_doc
print f.func_name
Functions in Python are objects.All objects in Python inherit from this base entity。
Each object in Python has a unique id. The id() function returns the object's id.
Objects may have attributes。
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13. functions
Objects can be stored in collections and passed to functions.
''' Here goes code'''
def f():
pass
def g():
pass
def h(f):
print id(f)
a = (f, g, h)
for i in a:
print i
h(f)
h(g)
''' Here goes result '''
<function f at 0x00000000020F0518>
<function g at 0x00000000020F0588>
<function h at 0x00000000020F05F8>
34538776
34538888
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13. Function redefinition
Python is dynamic in nature. It is possible to redefine an already defined function.
''' Here goes code'''
def showMessage(msg):
print msg
showMessage("Ready.")
def showMessage(msg):
print strftime("%H:%M:%S", gmtime()),
print msg
showMessage("Processing.")
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14. Python 函数参数引用（传值/传址）/copy/deepcopy
精简版：
传值：被调函数局部变量改变不会影响主调函数局部变量
传址：被调函数局部变量改变会影响主调函数局部变量
Python参数传递方式：传递对象引用（传值和传址的混合方式），如果是数字，字符串，元组则传值；如果是列表，字典则传址；
copy使用场景：列表或字典，且内部元素为数字，字符串或元组
deepcopy使用场景：列表或字典，且内部元素包含列表或字典
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15. Global and local variables
By default, we can get the contents of a global variable inside the body of a function. But if we want to change a global variable in a function, we must use the global keyword.
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16 .Anonymous functions
''' Here goes code'''
y = 6
z = lambda x: x * y
print z(8)
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17. r/R Raw String
print r'\n' prints \n and print R'\n'prints \n
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18. removing elements in lists
18.1 n.pop(index) will remove the item at index from the list and return it to you
18.2 n.remove(item) will remove the actual item if it finds it
18.3 del(n[1]) is like .pop in that it will remove the item at the given index, but it won't return it
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19. while/else
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.
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20. Multiple lists
zip will create pairs of elements when passed two lists, and will stop at the end of the shorter list.
zip can handle three or more lists as well!
# Here goes code
for a, b in zip(list_a, list_b):
# Add your code here!
print max(a,b)
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21. global variable
You can use a global variable in other functions by declaring it as global in each function that assigns to it:
#Here goes code
globvar = 0
def set_globvar_to_one():
global globvar # Needed to modify global copy of globvar
globvar = 1
def print_globvar():
print(globvar) # No need for global declaration to read value of globvar
set_globvar_to_one()
print_globvar() # Prints 1
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22. 字符串反转
方法一，使用[::-1]：
s = 'python'
print s[::-1]
方法二，使用reverse()方法：
l = list(s)
l.reverse()
print ''.join(l)
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