[Python] Some Tips

Posted on by Brad Lee 3 mins read
Table of Contents

原帖: Python开发的10个小贴士

Tips 1: List Comprehension

[Python] Comprehensions Concepts

Tips 2: Directly Get Value of List object

Bad

bag = [1, 2, 3, 4, 5]
for i in range(len(bag)):
    print(bag[i])

Good

bag = [1, 2, 3, 4, 5]
for i in bag:
    print(i)

Get the index of list itme if you need

bag = [1, 2, 3, 4, 5]
for index, element in enumerate(bag):
    print(index, element)

Tips 3: Exchange the value of variables

Bad

a = 5
b = 10

tmp = a
a = b
b = tmp

Good

a = 5
b = 10
a, b = b, a

Tips 4: Initialize List

General

bag = []
for _ in range(10):
    bag.append(0)

Good

bag = [0]*10

Trap: Shallow copy

bag = [[0]] * 5

The result is NOT what we want.

bag[0][0] = 1

The result of bag will be [[1],[1],[1],[1],[1]].

bag = [[0] for _ in range(5)]
bag[0][0] = 1

The result of bag will be [[1],[0],[0],[0],[0]]

Tips 5: String Access

Bad

name = "Raymond"
age = 22
born_in = "Oakland, CA"
string = "Hello my name is " + name + "and I'm " + str(age) + " years old. I was born in " + born_in + "."
print(string)

Good

name = "Raymond"
age = 22
born_in = "Oakland, CA"
string = "Hello my name is {0} and I'm {1} years old. I was born in {2}.".format(name, age, born_in)
print(string)

Tips 6: Return tuples

Bad

def binary():
    return 0, 1

result = binary()
zero = result[0]
one = zero[1]

Good

zero, one = binary()

or

zero, _ = binary()

Tips 7: Get the value of Dict by dict.get() with default value.

Not Good

一般對於不存在的 dictionary key/value, 為了避免 KeyError, 會用如下的寫法完成 error handling.

countr = {}
bag = [2, 3, 1, 2, 5, 6, 7, 9, 2, 7]
for i in bag:
    if i in countr:
        countr[i] += 1
    else:
        countr[i] = 1

for i in range(10):
    if i in countr:
        print("Count of {}: {}".format(i, countr[i]))
    else:
        print("Count of {}: {}".format(i, 0))

Good

可以使用 dict.get(key, defaultValue) 來完成避免 KeyError 的 error handling.

countr = {}
bag = [2, 3, 1, 2, 5, 6, 7, 9, 2, 7]
for i in bag:
    countr[i] = countr.get(i, 0) + 1

for i in range(10):
    print("Count of {}: {}".format(i, countr.get(i, 0)))

Tips 8: List Slice

List [ Start: Stop: Step ]

取前 5 個元素

bag = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
for elem in bag[:5]:
    print(elem)

取後 5 個元素

bag = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
for elem in bag[-5:]:
    print(elem)

每隔 2 個取一個元素

bag = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
for elem in bag[::2]:
    print(elem)

從後頭數過來取元素

bag = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
for elem in bag[::-1]:
    print(elem)