Must-see Python tips for beginners

The following are some Python practical tips and tools that I have collected in recent years. I hope they can be helpful to you.

ExchangeVariables

x = 6
y = 5
x, y = y, x
print x
>>> 5
print y
>>> 6

if Statement in line

print "Hello" if True else "World"
>>> Hello

Connection

The last one below This method is very cool when binding two objects of different types.

nfc = ["Packers", "49ers"]
afc = ["Ravens", "Patriots"]
print nfc + afc
>>> ['Packers', '49ers', 'Ravens', 'Patriots']
 
print str(1) + " world"
>>> 1 world
 
print `1` + " world"
>>> 1 world
 
print 1, "world"
>>> 1 world
print nfc, 1
>>> ['Packers', '49ers'] 1

Number skills

#除后向下取整
print 5.0//2
>>> 2
# 2的5次方
print 2**5
>> 32

Pay attention to the division of floating point numbers

print .3/.1
>>> 2.9999999999999996
print .3//.1
>>> 2.0

Numerical comparison

This is one of the few languages ​​I have seen that is so awesome Simple method

x = 2
if 3 > x > 1:
   print x
>>> 2
if 1  0:
   print x
>>> 2

Iterate two lists simultaneously

nfc = ["Packers", "49ers"]
afc = ["Ravens", "Patriots"]
for teama, teamb in zip(nfc, afc):
     print teama + " vs. " + teamb
>>> Packers vs. Ravens
>>> 49ers vs. Patriots

Indexed list iteration

teams = ["Packers", "49ers", "Ravens", "Patriots"]
for index, team in enumerate(teams):
    print index, team
>>> 0 Packers
>>> 1 49ers
>>> 2 Ravens
>>> 3 Patriots

List comprehension

Given a list, we can The method of brushing to select an even number list:

numbers = [1,2,3,4,5,6]
even = []
for number in numbers:
    if number%2 == 0:
        even.append(number)

changes to the following:

numbers = [1,2,3,4,5,6]
even = [number for number in numbers if number%2 == 0]

Isn’t it awesome, haha.

Dictionary comprehension

Similar to list comprehension, a dictionary can do the same job:

teams = ["Packers", "49ers", "Ravens", "Patriots"]
print {key: value for value, key in enumerate(teams)}
>>> {'49ers': 1, 'Ravens': 2, 'Patriots': 3, 'Packers': 0}

Initialize the value of the list

items = [0]*3
print items
>>> [0,0,0]

Convert the list to String

teams = ["Packers", "49ers", "Ravens", "Patriots"]
print ", ".join(teams)
>>> 'Packers, 49ers, Ravens, Patriots'

Get elements from dictionary

I admit that the try/except code is not elegant, but here is a simple method, try to find the key in the dictionary, if not found The corresponding alue will be set to its variable value using the second parameter.

data = {'user': 1, 'name': 'Max', 'three': 4}
try:
   is_admin = data['admin']
except KeyError:
   is_admin = False

Replace with this:

data = {'user': 1, 'name': 'Max', 'three': 4}
is_admin = data.get('admin', False)

Getting a subset of a list

Sometimes, you only need some of the elements in a list. Here are some ways to get a subset of a list.

x = [1,2,3,4,5,6]
#前3个
print x[:3]
>>> [1,2,3]
#中间4个
print x[1:5]
>>> [2,3,4,5]
#最后3个
print x[3:]
>>> [4,5,6]
#奇数项
print x[::2]
>>> [1,3,5]
#偶数项
print x[1::2]
>>> [2,4,6]

60 characters to solve FizzBuzz

Some time ago Jeff Atwood promoted a simple programming exercise called FizzBuzz. The question is quoted as follows:

Write a The program prints the numbers 1 to 100, replacing the number with "Fizz" for multiples of 3, "Buzz" for multiples of 5, and "FizzBuzz" for numbers that are both multiples of 3 and 5.

Here is a short, interesting way to solve this problem:

for x in range(101): print"fizz"[x%3*4::]+"buzz"[x%5*4::] or x

Collection

In addition to python's built-in

data type, in the collection module It also includes some special use cases, where Counter is very useful. If you participated in this year's Facebook HackerCup, you can even find its practicality.

from collections import Counter
print Counter("hello")
>>> Counter({'l': 2, 'h': 1, 'e': 1, 'o': 1})

Iteration tool

Like the collections library, there is also a library called itertools, which can really solve certain problems efficiently. One of the use cases is to find all combinations, which can tell you all the impossible combinations of elements in a group

from itertools import combinations
teams = ["Packers", "49ers", "Ravens", "Patriots"]
for game in combinations(teams, 2):
    print game
>>> ('Packers', '49ers')
>>> ('Packers', 'Ravens')
>>> ('Packers', 'Patriots')
>>> ('49ers', 'Ravens')
>>> ('49ers', 'Patriots')
>>> ('Ravens', 'Patriots')
False == True

This is a very interesting thing compared to practical technology. In python, True and False is a global variable, so:

False = True
if False:
   print "Hello"
else:
   print "World"
>>> Hello

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