Python入门：变量、字符串与基础操作详解

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本篇Python基本入门小抄旨在为初学者提供一个简洁且全面的指南，涵盖了Python编程语言的基础知识。内容主要分为以下几个部分： 1. 变量与字符串： - 变量是存储数据的容器，它们用来保存各种类型的数据，如整数、浮点数、布尔值等。 - 字符串是一系列字符的序列，用单引号或双引号包围。例如，`msg="Helloworld!"`展示了如何定义并打印一个字符串。 - 字符串操作包括字符串连接，如将`first_name`和`last_name`拼接成全名：`full_name=first_name+' '+last_name`。 2. 列表： - 列表是一种有序的数据结构，可以存储不同类型的数据，并通过索引访问元素，如`first_bike = bikes[0]`获取列表中的第一个元素。 - 使用循环遍历列表，如`for bike in bikes: print(bike)`。 - 列表操作还包括添加元素（如`bikes.append()`）和创建切片（如`first_two=finishers[:2]`）。 - 列表的复制可以通过切片实现，如`copy_of_bikes = bikes[:]`。 3. 数组（在Python中通常不直接使用这个术语，但可能是指列表）： - 可以使用列表来创建数字数组，如生成1到10的平方数组：`squares = [x**2 for x in range(1,11)]`，这是列表推导式的一个应用。 4. 元组： - 元组类似于列表，但元素不可变，这意味着一旦创建就不能修改。例如，`finishers = ('sam', 'bob', 'ada', 'bea')`定义了一个元组。通过学习这些基础知识，初学者能够掌握Python编程的基本语法和数据结构，为进一步学习函数、模块、面向对象编程等更高级的主题打下坚实的基础。此外，本文档还提供了实践操作示例，有助于新手通过动手操作加深理解。对于希望快速入门Python的读者来说，这是一份宝贵的参考材料。

Use curly braces to define a dictionary. Use colons to

connect keys and values, and use commas to separate

individual key-value pairs.

Making a dictionary

alien_0 = {'color': 'green', 'points': 5}

Covers Python 3 and Python 2

You can loop through a dictionary in three ways: you can

loop through all the key-value pairs, all the keys, or all the

values.

A dictionary only tracks the connections between keys

and values; it doesn't track the order of items in the

dictionary. If you want to process the information in order,

you can sort the keys in your loop.

Looping through all key-value pairs

# Store people's favorite languages.

fav_languages = {

'jen': 'python',

'sarah': 'c',

'edward': 'ruby',

'phil': 'python',

}

# Show each person's favorite language.

for name, language in fav_languages.items():

print(name + ": " + language)

Looping through all the keys

# Show everyone who's taken the survey.

for name in fav_languages.keys():

print(name)

Looping through all the values

# Show all the languages that have been chosen.

for language in fav_languages.values():

print(language)

Looping through all the keys in order

# Show each person's favorite language,

# in order by the person's name.

for name in sorted(fav_languages.keys()):

print(name + ": " + language)

Python's dictionaries allow you to connect pieces of

related information. Each piece of information in a

dictionary is stored as a key-value pair. When you

provide a key, Python returns the value associated

with that key. You can loop through all the key-value

pairs, all the keys, or all the values.

To access the value associated with an individual key give

the name of the dictionary and then place the key in a set of

square brackets. If the key you're asking for is not in the

dictionary, an error will occur.

You can also use the get() method, which returns None

instead of an error if the key doesn't exist. You can also

specify a default value to use if the key is not in the

dictionary.

Getting the value associated with a key

alien_0 = {'color': 'green', 'points': 5}

print(alien_0['color'])

print(alien_0['points'])

Getting the value with get()

alien_0 = {'color': 'green'}

alien_color = alien_0.get('color')

alien_points = alien_0.get('points', 0)

print(alien_color)

print(alien_points)

You can modify the value associated with any key in a

dictionary. To do so give the name of the dictionary and

enclose the key in square brackets, then provide the new

value for that key.

Modifying values in a dictionary

alien_0 = {'color': 'green', 'points': 5}

print(alien_0)

# Change the alien's color and point value.

alien_0['color'] = 'yellow'

alien_0['points'] = 10

print(alien_0)

You can remove any key-value pair you want from a

dictionary. To do so use the del keyword and the dictionary

name, followed by the key in square brackets. This will

delete the key and its associated value.

Deleting a key-value pair

alien_0 = {'color': 'green', 'points': 5}

print(alien_0)

del alien_0['points']

print(alien_0)

You can store as many key-value pairs as you want in a

dictionary, until your computer runs out of memory. To add

a new key-value pair to an existing dictionary give the name

of the dictionary and the new key in square brackets, and

set it equal to the new value.

This also allows you to start with an empty dictionary and

add key-value pairs as they become relevant.

Adding a key-value pair

alien_0 = {'color': 'green', 'points': 5}

alien_0['x'] = 0

alien_0['y'] = 25

alien_0['speed'] = 1.5

Adding to an empty dictionary

alien_0 = {}

alien_0['color'] = 'green'

alien_0['points'] = 5

Try running some of these examples on pythontutor.com.

You can find the number of key-value pairs in a dictionary.

Finding a dictionary's length

num_responses = len(fav_languages)

没有找到则返回none,

没有找到则返回0

It's sometimes useful to store a set of dictionaries in a list;

this is called nesting.

Storing dictionaries in a list

# Start with an empty list.

users = []

# Make a new user, and add them to the list.

new_user = {

'last': 'fermi',

'first': 'enrico',

'username': 'efermi',

}

users.append(new_user)

# Make another new user, and add them as well.

new_user = {

'last': 'curie',

'first': 'marie',

'username': 'mcurie',

}

users.append(new_user)

# Show all information about each user.

for user_dict in users:

for k, v in user_dict.items():

print(k + ": " + v)

print("\n")

You can also define a list of dictionaries directly,

without using append():

# Define a list of users, where each user

# is represented by a dictionary.

users = [

{

'last': 'fermi',

'first': 'enrico',

'username': 'efermi',

{

'last': 'curie',

'first': 'marie',

'username': 'mcurie',

]

# Show all information about each user.

for user_dict in users:

for k, v in user_dict.items():

print(k + ": " + v)

print("\n")

Storing a list inside a dictionary alows you to associate

more than one value with each key.

Storing lists in a dictionary

# Store multiple languages for each person.

fav_languages = {

'jen': ['python', 'ruby'],

'sarah': ['c'],

'edward': ['ruby', 'go'],

'phil': ['python', 'haskell'],

}

# Show all responses for each person.

for name, langs in fav_languages.items():

print(name + ": ")

for lang in langs:

print("- " + lang)

More cheat sheets available at

Standard Python dictionaries don't keep track of the order

in which keys and values are added; they only preserve the

association between each key and its value. If you want to

preserve the order in which keys and values are added, use

an OrderedDict.

Preserving the order of keys and values

from collections import OrderedDict

# Store each person's languages, keeping

# track of who respoded first.

fav_languages = OrderedDict()

fav_languages['jen'] = ['python', 'ruby']

fav_languages['sarah'] = ['c']

fav_languages['edward'] = ['ruby', 'go']

fav_languages['phil'] = ['python', 'haskell']

# Display the results, in the same order they

# were entered.

for name, langs in fav_languages.items():

print(name + ":")

for lang in langs:

print("- " + lang)

You can store a dictionary inside another dictionary. In this

case each value associated with a key is itself a dictionary.

Storing dictionaries in a dictionary

users = {

'aeinstein': {

'first': 'albert',

'last': 'einstein',

'location': 'princeton',

'mcurie': {

'first': 'marie',

'last': 'curie',

'location': 'paris',

}

for username, user_dict in users.items():

print("\nUsername: " + username)

full_name = user_dict['first'] + " "

full_name += user_dict['last']

location = user_dict['location']

print("\tFull name: " + full_name.title())

print("\tLocation: " + location.title())

Nesting is extremely useful in certain situations. However,

be aware of making your code overly complex. If you're

nesting items much deeper than what you see here there

are probably simpler ways of managing your data, such as

using classes.

You can use a loop to generate a large number of

dictionaries efficiently, if all the dictionaries start out with

similar data.

A million aliens

aliens = []

# Make a million green aliens, worth 5 points

# each. Have them all start in one row.

for alien_num in range(1000000):

new_alien = {}

new_alien['color'] = 'green'

new_alien['points'] = 5

new_alien['x'] = 20 * alien_num

new_alien['y'] = 0

aliens.append(new_alien)

# Prove the list contains a million aliens.

num_aliens = len(aliens)

print("Number of aliens created:")

print(num_aliens)

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