Python基础学习笔记之一

一、变量、运算和表达式

首先看一下这个表格：



需要注意的是：5/2=2; 5.0/2=2.5;

###################################
### compute #######
# raw_input() get input from keyboard to string type
# So we should transfer to int type
# Some new support computing type:
# and or not in is < <= != == | ^ & << + - / % ~
print 'Please input a number:'
number = int(raw_input)
number += 1
print number**2  # ** means^
print number and 1
print number or 1
print not number

二、数据类型

1、数字

通常的int、long、float等等都支持。而且会看你的具体数字来定义变量的类型。

如下：

### type of value #######
# int, long, float
# do not need to define the type of value, python will
# do this according to your value
num = 1    # stored as int type
num = 111111111   # stored as long int type
num = 1.0    # stored as float type
num = 12L    # L stands for long type
num = 1 + 12j   # j stands for complex type
num = '1'    # string type

2、字符串

单引号，双引号和三引号都可以用来定义字符串。三引号可以定义特别格式的字符串。字符串作为一种序列类型，支持像Matlab一样的索引访问和切片访问。

num = "1"  # string type
num = "Let's go" # string type
num = "He's \"old"" # string type
mail = """
xiaoyi:
hello
I am you!
""" # special string format
string = 'xiaoyi'  # get value by index
copy = string[0] + string[1] + string[2:6] # note: [2:6] means [2 5] or[2 6)
copy = string[:4] # start from 1
copy = string[2:] # to end
copy = string[::1] # step is 1, from start to end
copy = string[::2] # start is 2
copy = string[-1] # means 'i', the last one
copy = string[-4:-2:-1] # means 'yoa', -1 step controls direction
memAddr = id(num) # id(num) get the memory address of num
type(num) # get the type of num

3、元祖

元祖tuple用()来定义。相当于一个可以存储不同类型数据的一个数组。可以用索引来访问，但需要注意的一点是，里边的元素不能被修改。

firstName = 'Zou'
lastName = 'Xiaoyi'
len(string) # the length
name = firstName + lastName # concatenate 2 string
firstName * 3 # repeat firstName 3 times
'Z' in firstName # check contain or not, return true
string = '123'
max(string)
min(string)
cmp(firstName, lastName) #return 1, -1 or 0

## tuple(or array? structure? cell?)
## define this type using ()
user = ("xiaoyi", 25, "male")
name = user[0]
age = user[1]
gender user[2]
t1 = () # empty tuple
t2 = (2, ) # when tuple has only one element, we should add a extra comma
user[1] = 26 # error!! the elements can not be changed
name, age, gender = user # can get three element respectively
a, b, c = (1, 2, 3)

4、列表

列表list用[]来定义，它和元祖的功能一样，不同的一点是，里边的元素可以修改。List是一个类，支持很多该类定义的方法，这些方法可以用来对list进行操作。

userList = ["xiaoyi", 25, "male"]
name = userList[0]
age = userList[1]
gender = userList[2]
userList[3] = 88888 # error! access out of range, this is different with Matlab
userList.append(8888) # add new elements
"male" in userList # search
userList[2] = 'female' # can modify the element (the memory address not change)
userList.remove(8888) #remove element
userList.remove(userList[2]) #remove element
del(userList[1]) # use system operation api
## help(list.append)
######## object and class ######
## object = property + method
## python treats anything as class, here the list type is a class,
## when we define a list "userList", so we got a object, and we use
## its method to operate the elements

5、字典

字典dictionary用{}来定义。它的优点是定义像key-value这种键值非的结构，就像struct结构体的功能一样。它也支持字典类型支持的方法进行创建和操作。

######## dictionary type ######
## define this type using {}
item = ['name', 'age', 'gender']
value = ['xiaoyi', '25', 'male']
zip(item, value) # zip() will produce a new list:
# [('name', 'xiaoyi'), ('age', '25'), ('gender', 'male')]
# but we can not define their corresponding relationship
# and we can define this relationship use dictionary type
# This can be defined as a key-value manner
# dic = {key1: value1, key2: value2, ...}, key and value can be any type
dic = {'name': 'xiaoyi', 'age': 25, 'gender': 'male'}
dic = {1: 'zou', 'age':25, 'gender': 'male'}
# and we access it like this: dic[key1], the key as a index
print dic['name']
print dic[1]
# another methods create dictionary
fdict = dict(['x', 1], ['y', 2]) # factory mode
ddict = {}.fromkeys(('x', 'y'), -1) # built-in mode, default value is the same which is none
# access by for circle
for key in dic
print key
print dic[key]

# add key or elements to dictionary, because dictionary is out of sequence,
# so we can directly and a key-value pair like this:
dic['tel'] = 88888
# update or delete the elements
del dic[1] # delete this key
dic.pop('tel') # show and delete this key
dic.clear() # clear the dictionary
del dic # delete the dictionary
dic.get(1) # get the value of key
dic.get(1, 'error') # return a user-define message if the dictionary do not contain the key
dic.keys()
dic.values()
dic.has_key(key)
# dictionary has many operations, please use help to check out

三、流程控制

在这块，Python与其它大多数语言有个非常不同的地方，Python语言使用缩进块来表示程序逻辑（其它大多数语言使用大括号等）。例如：

if age < 21:

print("你不能买酒。")

print("不过你能买口香糖。")

print("这句话处于if语句块的外面。")

这个代码相当于c语言的：

if (age < 21)

{

print("你不能买酒。")

print("不过你能买口香糖。")

}

print("这句话处于if语句块的外面。")

可以看到，Python语言利用缩进表示语句块的开始和退出（Off-side规则），而非使用花括号或者某种关键字。增加缩进表示语句块的开始（注意前面有个:号），而减少缩进则表示语句块的退出。根据PEP的规定，必须使用4个空格来表示每级缩进（不清楚4个空格的规定如何，在实际编写中可以自定义空格数，但是要满足每级缩进间空格数相等）。使用Tab字符和其它数目的空格虽然都可以编译通过，但不符合编码规范。

为了使我们自己编写的程序能很好的兼容别人的程序，我们最好还是按规范来，用四个空格来缩减（注意，要么都是空格，要是么都制表符，千万别混用）。

1、 if-else

if-else 用来判断一些条件，以执行满足某种条件的代码。

######## procedure control #####
## if else
if expression: # bool type and do not forget the colon
statement(s)# use four space key

if expression:
statement(s) # error!!!! should use four space key

if 1<2:
print 'ok, ' # use four space key
print 'yeah' # use the same number of space key

if True: # true should be big letter True
print 'true'

def fun():
return 1

if fun():
print 'ok'
else:
print 'no'

con = int(raw_input('please input a number:'))
if con < 2:
print 'small'
elif con > 3:
print 'big'
else:
print 'middle'

if 1 < 2:
if 2 < 3:
print 'yeah'
else:
print 'no'
print 'out'
else:
print 'bad'

if 1<2 and 2<3 or 2 < 4 not 0: # and, or, not
print 'yeah'

2、 for

for的作用是循环执行某段代码。还可以用来遍历我们上面所提到的序列类型的变量。

######## procedure control #####
## for
for iterating_val in sequence:
statements(s)
# sequence type can be string, tuple or list

for i in "abcd":
print i

for i in [1, 2, 3, 4]:
print i

# range(start, end, step), if not set step, default is 1,
# if not set start, default is 0, should be noted that it is [start, end), not [start, end]
range(5) # [0, 1, 2, 3, 4]
range(1, 5) # [1, 2, 3, 4]
range(1, 10, 2) # [1, 3, 5, 7, 9]
for i in range(1, 100, 1):
print i

# ergodic for basis sequence
fruits = ['apple', 'banana', 'mango']
for fruit in range(len(fruits)):
print 'current fruit: ', fruits[fruit]

# ergodic for dictionary
dic = {1: 111, 2: 222, 5: 555}
for x in dic:
print x, ': ', dic[x]

dic.items() # return [(1, 111), (2, 222), (5, 555)]
for key,value in dic.items(): # because we can: a,b=[1,2]
print key, ': ', value
else:
print 'ending'

################################
import time
# we also can use: break, continue to control process
for x in range(1, 11):
print x
time.sleep(1) # sleep 1s
if x == 3:
pass # do nothing
if x == 2:
continue
if x == 6:
break
if x == 7:
exit() # exit the whole program
print '#'*50

3、while

while的用途也是循环。它首先检查在它后边的循环条件，若条件表达式为真，它就执行冒号后面的语句块，然后再次测试循环条件，直至为假。冒号后面的缩近语句块为循环体。

######## procedure control #####
## while
while expression:
statement(s)

while True:
print 'hello'
x = raw_input('please input something, q for quit:')
if x == 'q':
break
else:
print 'ending'

4、switch

其实Python并没有提供switch结构，但我们可以通过字典和函数轻松的进行构造。例如：

## switch ####
## this structure do not support by python
## but we can implement it by using dictionary and function
## cal.py ##
#!/usr/local/python

from __future__ import division
# if used this, 5/2=2.5, 6/2=3.0

def add(x, y):
return x + y
def sub(x, y):
return x - y
def mul(x, y):
return x * y
def div(x, y):
return x / y

operator = {"+": add, "-": sub, "*": mul, "/": div}
operator["+"](1, 2) # the same as add(1, 2)
operator["%"](1, 2) # error, not have key "%", but the below will not
operator.get("+")(1, 2) # the same as add(1, 2)

def cal(x, o, y):
print operator.get(o)(x, y)
cal(2, "+", 3)
# this method will effect than if-else

四、函数

1、自定义函数

在Python中，使用def语句来创建函数：

######## function #####
def functionName(parameters): # no parameters is ok
bodyOfFunction

def add(a, b):
return a+b # if we do not use a return, any defined function will return default None

a = 100
b = 200
sum = add(a, b)

##### function.py #####
#!/usr/bin/python
#coding:utf8  # support chinese
def add(a = 1, b = 2): # default parameters
return a+b  # can return any type of data
# the followings are all ok
add()
add(2)
add(y = 1)
add(3, 4)

###### the global and local value #####
## global value: defined outside any function, and can be used
##              in anywhere, even in functions, this should be noted
## local value: defined inside a function, and can only be used
##              in its own function
## the local value will cover the global if they have the same name
val = 100 # global value
def fun():
print val # here will access the val = 100
print val # here will access the val = 100, too

def fun():
a = 100 # local value
print a
print a # here can not access the a = 100

def fun():
global a = 100 # declare as a global value
print a

print a # here can not access the a = 100, because fun() not be called yet
fun()
print a # here can access the a = 100

############################
## other types of parameters
def fun(x):
print x
# the follows are all ok
fun(10) # int
fun('hello') # string
fun(('x', 2, 3))  # tuple
fun([1, 2, 3])    # list
fun({1: 1, 2: 2}) # dictionary

## tuple
def fun(x, y):
print "%s : %s" % (x,y) # %s stands for string
fun('Zou', 'xiaoyi')
tu = ('Zou', 'xiaoyi')
fun(*tu)    # can transfer tuple parameter like this

## dictionary
def fun(name = "name", age = 0):
print "name: %s" % name
print "age: " % age
dic = {name: "xiaoyi", age: 25} # the keys of dictionary should be same as fun()
fun(**dic) # can transfer dictionary parameter like this
fun(age = 25, name = 'xiaoyi') # the result is the same
## the advantage of dictionary is can specify value name

#############################
## redundancy parameters ####
## the tuple
def fun(x, *args): # the extra parameters will stored in args as tuple type
print x
print args
# the follows are ok
fun(10)
fun(10, 12, 24) # x = 10, args = (12, 24)

## the dictionary
def fun(x, **args): # the extra parameters will stored in args as dictionary type
print x
print args
# the follows are ok
fun(10)
fun(x = 10, y = 12, z = 15) # x = 10, args = {'y': 12, 'z': 15}

# mix of tuple and dictionary
def fun(x, *args, **kwargs):
print x
print args
print kwargs
fun(1, 2, 3, 4, y = 10, z = 12) # x = 1, args = (2, 3, 4), kwargs = {'y': 10, 'z': 12}

2、Lambda函数

Lambda函数用来定义一个单行的函数，其便利在于：

## lambda function ####
## define a fast single line function
fun = lambda x,y : x*y # fun is a object of function class
fun(2, 3)
# like
def fun(x, y):
return x*y

## recursion
# 5=5*4*3*2*1, n!
def recursion(n):
if n > 0:
return n * recursion(n-1) ## wrong

def mul(x, y):
return x * y
numList = range(1, 5)
reduce(mul, numList) # 5! = 120
reduce(lambda x,y : x*y, numList) # 5! = 120, the advantage of lambda function avoid defining a function

### list expression
numList = [1, 2, 6, 7]
filter(lambda x : x % 2 == 0, numList)
print [x for x in numList if x % 2 == 0] # the same as above
map(lambda x : x * 2 + 10, numList)
print [x * 2 + 10 for x in numList] # the same as above

3、Python内置函数

Python内置了很多函数，他们都是一个个的.py文件，在python的安装目录可以找到。弄清它有那些函数，对我们的高效编程非常有用。这样就可以避免重复的劳动了。下面也只是列出一些常用的：

## built-in function of python ####
## if do not how to use, please use help()
abs, max, min, len, divmod, pow, round, callable,
isinstance, cmp, range, xrange, type, id, int()
list(), tuple(), hex(), oct(), chr(), ord(), long()

callable # test a function whether can be called or not, if can, return true
# or test a function is exit or not

isinstance # test type
numList = [1, 2]
if type(numList) == type([]):
print "It is a list"
if isinstance(numList, list): # the same as above, return true
print "It is a list"

for i in range(1, 10001) # will create a 10000 list, and cost memory
for i in xrange(1, 10001)# do not create such a list, no memory is cost

## some basic functions about string
str = 'hello world'
str.capitalize() # 'Hello World', first letter transfer to big
str.replace("hello", "good") # 'good world'
ip = "192.168.1.123"
ip.split('.') # return ['192', '168', '1', '123']
help(str.split)

import string
str = 'hello world'
string.replace(str, "hello", "good") # 'good world'

## some basic functions about sequence
len, max, min
# filter(function or none, sequence)
def fun(x):
if x > 5:
return True
numList = [1, 2, 6, 7]
filter(fun, numList) # get [6, 7], if fun return True, retain the element, otherwise delete it
filter(lambda x : x % 2 == 0, numList)
# zip()
name = ["me", "you"]
age = [25, 26]
tel = ["123", "234"]
zip(name, age, tel) # return a list: [('me', 25, '123'), ('you', 26, '234')]
# map()
map(None, name, age, tel) # also return a list: [('me', 25, '123'), ('you', 26, '234')]
test = ["hello1", "hello2", "hello3"]
zip(name, age, tel, test) # return [('me', 25, '123', 'hello1'), ('you', 26, '234', 'hello2')]
map(None, name, age, tel, test) # return [('me', 25, '123', 'hello1'), ('you', 26, '234', 'hello2'), (None, None, None, 'hello3')]
a = [1, 3, 5]
b = [2, 4, 6]
def mul(x, y):
return x*y
map(mul, a, b) # return [2, 12, 30]
# reduce()
reduce(lambda x, y: x+y, [1, 2, 3, 4, 5]) # return ((((1+2)+3)+4)+5)