Learning Python(1)

The Zen of Python

The Zen of Python, by Tim Peters

Beautiful is better than ugly.
Explicit is better than implicit.
Simple is better than complex.
Complex is better than complicated.
Flat is better than nested.
Sparse is better than dense.
Readability counts.
Special cases aren't special enough to break the rules.
Although practicality beats purity.
Errors should never pass silently.
Unless explicitly silenced.
In the face of ambiguity, refuse the temptation to guess.
There should be one-- and preferably only one --obvious way to do it.
Although that way may not be obvious at first unless you're Dutch.
Now is better than never.
Although never is often better than right now.
If the implementation is hard to explain, it's a bad idea.
If the implementation is easy to explain, it may be a good idea.
Namespaces are one honking great idea -- let's do more of those!

Variation

• string

#Wrong! Python intepreter don't know 18 is a int or two char
#TypeError: can only concatenate str (not "int") to str
print("Happy "+18+" years old")
#Yes!Using str()
print("Happy "+str(18)+" years old")

List

• All list is dynamical, like "vector" in cpp. There is no native "Arrays". This is a great idea!

moto=['yamaha','suzuki']
#add to rear
moto.append('honda')
#delete
del moto[0]
moto.pop()
moto.remove('yamaha')
print(moto)
['yamaha', 'suzuki', 'honda']

• But there will the occasion that list index out of range.
• Using index -1 to acess the last of the element, convenient!

handle list

• traverse

motos=['yamaha','suzuki','suzuki']
for moto in motos:
    print(moto)

• indent is the identical element for loop !
• use range() to loop

#equals for(int i;i<5;i++) in c++
for value in range(1,5):
    print(value)
#1 2 3 4 

• use list(range()) to get radixed list

numbers=list(range(2,11,2))
print(numbers)
#[2, 4, 6, 8, 10]

• number list function

>>>digits=[1,2,3,4,5,6]
>>>min(digits)
0
>>>max(digits)
6
>>>sum(digits)
21

• list analysis

#It's so amazing!
squares = [value**2 for value in range(1,11)]
print(squares)

• slice of list

numlists=[1,2,3,4,5,6,7,8,9,10]
print(numlists[0:3])
print(numlists[:3])
print(numlists[3:])
print(numlists[-3:])
#[1, 2, 3]
#[1, 2, 3]
#[4, 5, 6, 7, 8, 9, 10]
#[8, 9, 10]

• copy a list

numlists=[1,2,3,4,5,6,7,8,9,10]
numlists2=numlists[:]
#numlist2=numlists Wrong! This expression will copy reference
numlists2.append(11)
print(numlists)
print(numlists2)
#[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
#[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]

• a tuple is a const list

tupleints=(10,20,30)

Control Flow

• diffrent expression

and <=> && in cpp
or <=> || in cpp

• check if list (not) contains one special element

>>>cars = ['audi','bmw','subaru','toyota']
>>>print('bmw' in cars)
True
>>>print('byd' in cars)
False

• elif=else if
• for loop is pass-by-value
• Handle the ocassion that there is no elements in list automatically:

cars = []
if cars:    
    print("I have cars!")
else:
    print("I don't have a car!")

• Dictionary

#also can be write in one line
aliens={
    'color':'green',
    'points':5
}
print(aliens['color'])
print(aliens['points'])
aliens['name']='holo' #add a key-value pair
print(aliens)
del aliens['color']

#also can be write in one line
aliens={
    'color':'green',
    'points':5
}
for key,value in aliens.items():
    print("the alien's "+key+" is value")
for item in aliens.items():
    print(item)
for key in aliens.keys():
    print(key)
for value in sorted(aliens.values()):
    print(value)

• input

message=input("tell me something:")
print("You say "+message)
tell me something:hello
You say hello

message=input("How old are you:")
#input is a string
#using type cast to get int
if int(message)>18:
    print('You are an adult!Welcome')

function

• argument with keyworld and default

def greet_user(Username='Noname'):
    """Hello"""
    print(Username+", Hello!")
greet_user(Username='David')

• argument is pass-by-value

def change(value):
    value=10
val=5
change(val)
print(val)
#5

• pass a list to modify its value

def change(values):
    for i in range(0,len(values)):
        values[i]=10
vals=[5]
change(vals)
print(vals)
#[10]

• pass a list but don't want the origin list to be modified:

def change(values):
    for value in values:
        value=10
vals=[5]
change(vals[:]) //safe mode, cost time
print(vals)
#[5]

• Varargs

def make_pizza(size,*toppings):
    #notice the order of args,*means toppings is a tuple.
    """make pizza with variable ingredients"""
    for top in toppings:
        print(top+" add!")
    print("This is a "+str(size)+"-inch pizza")
make_pizza("mushroom",'peppers','cheese')
#mushroom add!
#peppers add!
#cheese add!

def build_profile(first,last,**userinfo):
    #key-value pair as varargs,use two '*'
    """创建用户信息"""
    profile={}
    profile['first_name']=first
    profile['last_name']=last
    for key,value in userinfo.items():
        profile[key]=value
    return profile
first='albert'
last='einstein'
profile=build_profile(first.title(),last.title(),
                      major='physics',
                      local='princeton')
print(profile)
#{'first_name': 'Albert', 'last_name': 'Einstein', 'major': 'physics', 'local': 'princeton'}

import modules

import mod as p
#别名
p.fun()

from mod import *
fun()   #fun is a function in mod