Detailed introduction to python types (type)

Empty (None)

None can be used to indicate that the value of a certain variable is missing, similar to null in other languages.

Like other null values: 0, [] and empty string, Boolean variables give False instead of True.

if None:print("None got interpreted as True")else:print("None got interpreted as False")

The result is:

None got interpreted as False

When a function does not return any value, it returns None:

def some_func():print("Hi")
var=some_func()print(var)

The result is:

##

Hi
None

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Dictionaries

A dictionary is a data structure that assigns keys to values. A list can be thought of as a dictionary with some range of integer keys.

Dictionaries can be indexed like lists, using square brackets, except that the square brackets are no longer subscripts, but keywords

ages={"Dave":24,"Mary":42,"John":58}print(ages["Dave"])print(ages["Mary"])

The result is:

24
42

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An error will occur when indexing a keyword that is not a dictionary. Dictionaries can store values ​​of any data type. An empty dictionary is "{}".

The keywords of the dictionary cannot be changed. Using a mutable object as a dictionary key will produce a TypeError.

bad_dict={
    [1,2,3]:"one two three"}

The result is:

TypeError: unhashable type: 'list'

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Dictionary Functions (Dictionary Functions)

Dictionary keys can be assigned different values. If there is no keyword, create a new keyword:

squares={1:1,2:4,3:"error",4:16}
squares[8]=64squares[3]=9print(squares)

The result is:

{1: 1, 2: 4, 3: 9, 4: 16, 8: 64}

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Check whether a keyword exists in the dictionary with in or not in just like in the list.

nums={1:"one",2:"two",3:"three"}print(1 in nums)print("three"in nums)print(4 not in nums)

The result is:

True
False
True

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get It is a very easy-to-use dictionary method that plays the same role as an index, but if the keyword is not found in the dictionary, it will return None instead of the error

paris={1:"apple","orange":[2,3,4],
    True:False,
    None:"True"}print(paris.get("orange"))print(paris.get(7))print(paris.get(12345,"not in dictionary"))

get The second parameter means that if the keyword cannot be found, this value will be returned.

The result is:

##

paris={1:"apple","orange":[2,3,4],
    True:False,
    None:"True"}print(paris.get("orange"))print(paris.get(7))print(paris.get(12345,"not in the dicrionary"))

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Tuples

Tuples are very similar to lists, but they cannot be changed. You can create a new tuple with parentheses, or without...:

words=("spam","eggs","sausages",)

words="spam","eggs","sausages",

空元组用()新建。

元组的运行速度比列表快

其他使用方法和列表类似。

列表切片(List Slices)

列表切片是一种检索列表值的高级方法。基本的切片方法是用两个被冒号分开的整数来索引列表。这样可以从旧列表返回一个新列表。

squares=[0,1,4,9,16,25,36,49,64,81]print(squares[2:6])print(squares[3:8])print(squares[0:1])

结果是：

[4, 9, 16, 25]
[9, 16, 25, 36, 49]
[0]

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跟range的参数相似，第一的下标的值会包括，但不包括第二个下标的值。

如果第一个下标省略，默认从头开始，

如果第二个下标省略，默认到结尾结束。

切片同样可以用于元组。

切片也有第三个参数，决定了步长。第一二个分别决定了开头与结尾。

squares=[0,1,4,9,16,25,36,49,64,81]
print(squares[:6:2])
print(squares[3::3])
print(squares[::3])

结果是：

[0, 4, 16]
[9, 36, 81]
[0, 9, 36, 81]

参数是复数的话就倒着走。-1是倒数第一，-2是倒数第二，第三个参数为负就会倒着切，这时候第一个参数和第二个参数就要倒着看了，也就是第二个参数变成了开始，第一个变成了结尾（因此-1会使整个列表倒序）

squares=[0,1,4,9,16,25,36,49,64,81]print(squares[:-1])print(squares[::-3])print(squares[-3::2])

结果是：

[0, 1, 4, 9, 16, 25, 36, 49, 64]
[81, 36, 9, 0]
[49, 81]

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列表解析(List Comprehensions)

这是一种快速创建遵循某些规则的列表的方法：

cubes=[i**3 for i in range(5)]print(cubes)

结果是：

[0, 1, 8, 27, 64]

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也可以包含if statement 加强限定条件。

evens=[i**2 for i in range(10) if i**2 % 2==0]print(evens)

结果是：

[0, 4, 16, 36, 64]

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evens=[i**2 for i in range(10) if i**2 % 2==0]print(evens)

结果是：

[0, 4, 16, 36, 64]

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range的范围过大会超出内存的容量引发MemoryError

String Formatting

为了使string和non-string结合，可以把non-string转化为string然后再连起来。

string formatting提供了一种方式，把non-string嵌入到string里，用string的format method来替换string里的参数。

nums=[4,5,6]
msg="Numbers:{0} {1} {2}".format(nums[0],nums[1],nums[2])print(msg)

format里的参数和{}里的参数是对应的。{}的参数是format()里参数的下标

参数被命名这种情况也是可以的：

a="{x},{y}".format(x=5,y=12)print(a)

结果是：

5,12

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Useful Functions

Python 内置了许多有用的函数

join ，用一个string充当分隔符把一个由string组成的列表连起来。

print(",".join(["spam","eggs","ham"]))

结果是：

spam,eggs,ham

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replace，用一个string 取代另一个。

print("Hello ME".replace("ME","world"))

结果是：

Hello world

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startwith和endwith，判断是否是由……开头或结束：

print("This is a sentence.".startswith("This"))print("This is a sentence.".endswith("sentence."))

结果是：

True
True

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lower和upper可以改变string的大小写

print("This is A sentence.".upper())print("THIS IS a SENTENCE..".lower())

结果是：

THIS IS A SENTENCE.
this is a sentence.

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split的作用于join 相反，他可以按某个string为分隔符将一串string分开并成为列表的形式。

print("apple,eggs,banana".split(","))

结果是：

['apple', 'eggs', 'banana']

 有关数学的一些函数有：最大值max,最小值min，绝对值abs,约等数round（第二个参数可以决定保留几位小数），对列表里的数求和用sum等：

print(min(1,2,3,4,5,6,7))print(max(1,2,3,4,5,6,7))print(abs(-98))print(round(78.632453434,4))print(sum([2.12121,23232323]))

结果是：

1
7
98
78.6325
23232325.12121

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all和any可以把列表当成参数，然后返回True或 False,

nums=[55,44,33,22,11]if all([i <56 for i in nums]):print("All smaller than 56.")

 

nums=[55,44,33,22,11]if any([i <22 for i in nums]):print("at least one is smaller than 22.")

all和any的区别是，all需要所有的值都满足，any只需要有一个满足就行了。

枚举(enumerate),字面意思，把列表中的值按顺序一个一个列出来。

nums=[55,44,33,22,11]for v in enumerate(nums):print(v)

结果是：

(0, 55)
(1, 44)
(2, 33)
(3, 22)
(4, 11)

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