Introducing techniques to Python beginners

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

Exchange variables

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

connect

The last way below 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 such a great shorthand method that I have rarely seen in many languages

x = 2
if 3 > x > 1:
   print x
>>> 2
if 1 < x > 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 brush out the even list method:

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

Convert 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 derivation

Similar to list comprehensions, dictionaries 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}

Initialization list value

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

Convert list to string

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

Get elements from the dictionary

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

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

替换诚这样：

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

Get a subset of the list

Sometimes, you only need part of 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 program that 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

gather

In addition to Python's built-in data types, the collection module also includes some special use cases. Counter is very practical in some situations. 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 tools

Like the collections library, there is also a library called itertools, which can really solve certain problems efficiently. One use case 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 are global variables, so:

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

If you have any other cool tricks, you can leave a message below, thanks for reading.

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