密歇根大学Python入门教程：探索数据

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"Python for Everybody" 是由 Charles Severance 主编的一本针对初学者的 Python 入门教程，它作为 EDX 平台上密歇根大学提供的一项课程资料，旨在帮助读者探索如何使用 Python 3 进行数据处理。这本书在 2016 年 7 月首次发布完全基于 Python 3.0 的版本，之前在 2015 年 12 月进行了初步的 Python 3.0 转换。该书的创作理念是“remix”，即在 Allen B. Downey、Jeff Elkner 等人的《Think Python: How to Think Like a Computer Scientist》一书的基础上进行改编和扩展。作者 Charles Severance 在追求学术成果的同时，选择了一个创新的方式，将已有资源进行再利用和整合，这在教育领域中体现了开源和共享的精神。书中覆盖了基础的编程概念，包括变量、数据类型、控制结构、函数、模块和包等，旨在引导读者理解和掌握 Python 语言的基本语法和逻辑思维。此外，由于具有 Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License（CC BY-NC-SA 3.0）的许可协议，这意味着读者可以免费学习并用于非商业目的，但需注明原作者和保留版权信息，且不允许进行商业销售或在修改后的作品中排除原作者的署名。 “Python for Everybody”不仅是学习编程入门的理想教材，还适合对数据分析感兴趣的人员，因为它强调了如何用 Python 处理和分析实际数据。通过本书的学习，读者不仅能建立起扎实的 Python 基础，还能培养解决问题和数据驱动决策的能力。对于想要进入 IT 行业或者提升 Python 技能的读者来说，这本书提供了丰富的实践案例和逐步指导，使学习过程更为直观和易于理解。同时，随着网络编程的章节，它还介绍了网络基础知识，为学员未来在网络环境中的应用打下基础。这是一本兼具理论和实践、适合不同背景人群的 Python 入门教程，它既体现了开放教育资源的优势，也为读者打开了通向 Python 世界的大门。无论是个人自学还是课堂教学，都能从中受益匪浅。

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4 CHAPTER 1. WHY SHOULD YOU LEARN TO WRITE PROGRAMS?

As a programmer, your job is to use and orchestrate each of these resources to

solve the problem that you need to solve and analyze the data you get from the

solution. As a programmer you will mostly be “talking” to the CPU and telling

it what to do next. Sometimes you will tell the CPU to use the main memory,

secondary memory, network, or the input/output devices.

Input and

Output

Devices

Software

Main

Memory

Central

Processing

Unit

What

Next?

Network

Secondary

Memory

Figure 1.4: Where Are You?

You need to be the person who answers the CPU’s “What next?” question. But it

would be very uncomfortable to shrink you down to 5mm tall and insert you into

the computer just so you could issue a command three billion times per second. So

instead, you must write down your instructions in advance. We call these stored

instructions a program and the act of writing these instructions down and getting

the instructions to be correct programming.

1.3 Understanding programming

In the rest of this book, we will try to turn you into a p er son who is skilled in the art

of programming. In the end you will be a programmer - perhaps not a professional

programmer, but at least you will have the skills to look at a data/information

analysis problem and develop a program to solve the problem.

In a sense, you need two skills to be a programmer:

• First, you need to know the programming language (Python) - you need to

know the vocabulary and the grammar. You need to be able to spell the

words in this new language properly and know how to construct well-formed

“sentences” in this new language.

• Second, you need to “tell a story”. In writing a story, you combine words

and sentences to convey an idea to the reader. There is a skill and art in

constructing the story, and skill in story writing is improved by doing some

writing and getting some feedback. In programming, our program is the

“story” and the problem you are trying to solve is the “idea”.

Once you learn one programming language such as Python, you will ﬁnd it much

easier to learn a second programming language such as JavaScript or C++. The

1.4. WORDS AND SENTENCES 5

new programming language has very diﬀerent vocabulary and grammar but the

problem-solving skills will be the same across all programming languages.

You will learn the “vocabulary” and “sentences” of Python pretty quickly. It will

take longer for you to be able to write a coherent program to solve a brand-new

problem. We teach programming much like we teach writing. We start reading

and explaining programs, then we write simple programs, and then we write in-

creasingly complex programs over time. At some point you “get your muse” and

see the patterns on your own and can see more naturally how to take a problem

and write a program that solves that problem. And once you get to that point,

programming becomes a very pleasant and creative process.

We start with the vocabulary and structure of Python programs. Be patient as

the simple examples remind you of when you started reading for the ﬁrst time.

1.4 Words and sentences

Unlike human languages, the Python vocabulary is actually pretty small. We call

this “vocabulary” the “reserved words”. These are words that have very special

meaning to Python. When Python sees these words in a Python program, they

have one and only one meaning to Python. Later as you write programs you will

make up your own words that have meaning to you called variables. You will have

great latitude in choosing your names for your variables, but you cannot use any

of Python’s reserved words as a name for a variable.

When we train a dog, we us e special words like “sit”, “stay”, and “fetch”. When

you talk to a dog and don’t use any of the reserved words, they just look at you

with a quizzical look on their face until you say a r eserved word. For example, if

you say, “I wish more people would walk to improve their overall health”, what

most dogs likely hear is, “blah blah blah walk blah blah blah blah.” That is because

“walk” is a reserved word in dog language. Many might suggest that the language

between humans and cats has no reserved words

The reserved words in the language where humans talk to Python include the

following:

and del global not with

as elif if or yield

assert else import pass

break except in raise

class finally is return

continue for lambda try

def from nonlocal while

That is it, and unlike a dog, Python is already completely trained. When you say

“try”, Python will try every time you say it without fail.

We will learn these reserved words and how they are used in good time, but for

now we will focus on the Python equivalent of “speak” (in human-to-dog language).

The nice thing about telling Python to speak is that we can even tell it what to

say by giving it a message in quotes:

http://xkcd.com/231/

6 CHAPTER 1. WHY SHOULD YOU LEARN TO WRITE PROGRAMS?

print(

'Hello world!')

And we have even written our ﬁrst syntactically correct Python sentence. Our

sentence starts with the function print followed by a string of text of our choosing

enclosed in single quotes. The strings in the print statements are enclosed in quotes.

Single quotes and double quotes do the same thing; most people use single quotes

except in cases like this where a single quote (which is also an apostrophe) appears

in the string.

1.5 Conversing with Python

Now that we have a word and a simple sentence that we know in Python, we need

to know how to start a conversation with Python to test our new language skills.

Before you can converse with Python, you must ﬁrst install the Python software

on your computer and learn how to start Python on your computer. That is too

much detail for this chapter so I suggest that you consult

www.py4e.com where

I have detailed instructions and screencasts of setting up and starting Python on

Macintosh and Windows systems. At some point, you will be in a terminal or

command window and you will type python and the Python interpreter will start

executing in interactive mode and appear somewhat as follows:

Python

3.5.1 (v3.5.1:37a07cee5969, Dec 6 2015, 01:54:25)

[MSC v

.1900 64 bit (AMD64)] on win32

Type

"help", "copyright", "credits" or "license" for more

information.

>>>

The >>> prompt is the Python interpreter’s way of asking you, “What do you want

me to do next?” Python is ready to have a conversation with you. All you have

to know is how to speak the Python language.

Let’s say for example that you did not know even the simplest Python language

words or sentences. You might want to use the standard line that astronauts use

when they land on a faraway planet and try to speak with the inhabitants of the

planet:

>>> I come in peace, please take me to your leader

File "<stdin>", line 1

I come in peace, please take me to your leader

SyntaxError: invalid syntax

>>>

This is not going so well. Unless you think of something quickly, the inhabitants

of the planet are likely to stab you with their spears, put you on a spit, roast you

over a ﬁre, and eat you for dinner.

Luckily you brought a copy of this book on your travels, and you thumb to this

very page and try again:

1.5. CONVERSING WITH PYTHON 7

>>> print('Hello world!')

Hello world!

This is looking much better, so you try to communicate some more:

>>> print('You must be the legendary god that comes from the sky')

You must be the legendary god that comes from the sky

>>> print('We have been waiting for you for a long time')

We have been waiting

for you for a long time

>>> print('Our legend says you will be very tasty with mustard')

Our legend says you will be very tasty

with mustard

>>> print 'We will have a feast tonight unless you say

File "<stdin>", line 1

print '

We will have a feast tonight unless you say

SyntaxError: Missing parentheses in call to 'print'

>>>

The conversation was going so well for a while and then you made the tiniest

mistake using the Python language and Python brought the spears back out.

At this point, you should also realize that while Python is amazingly complex and

powerful and very picky about the syntax you use to communicate with it, Python

is not intelligent. You are really just having a conversation with yourself, but using

proper syntax.

In a sense, when you use a program written by someone else the conversation is

between you and those other programmers with Python acting as an intermediary.

Python is a way for the creators of programs to express how the conversation is

supposed to proceed. And in just a few more chapters, you will be one of those

programmers using Python to talk to the users of your program.

Before we leave our ﬁrst conversation with the Python interpreter, you should prob-

ably know the proper way to say “good-bye” when interacting with the inhabitants

of Planet Python:

>>> good-bye

Traceback (most recent call last):

File

"<stdin>", line 1, in <module>

NameError: name 'good' is not defined

>>> if you don't mind, I need to leave

File "<stdin>", line 1

if you don'

t mind, I need to leave

SyntaxError: invalid syntax

>>> quit()

You will notice that the error is diﬀerent for the ﬁrst two incorrect attempts. The

second error is diﬀerent because if is a reserved word and Python saw the reserved

word and thought we were trying to say something but got the syntax of the

sentence wrong.

8 CHAPTER 1. WHY SHOULD YOU LEARN TO WRITE PROGRAMS?

The proper way to say “good-bye” to Python is to enter quit() at the interactive

chevron >>> prompt. It would have probably taken you quite a while to guess that

one, so having a book handy probably will turn out to be helpful.

1.6 Terminology: Interpreter and compiler

Python is a high-level language intended to be relatively straightforward for hu-

mans to read and write and for computers to read and process. Other high-level

languages include Java, C++, PHP, Ruby, Basic, Perl, JavaScript, and many more.

The actual hardware inside the Central Processing Unit (CPU) does not under-

stand any of these high-level languages.

The CPU understands a language we call machine language. Machine language is

very simple and frankly very tiresome to write because it is represented all in zeros

and ones:

001010001110100100101010000001111

11100110000011101010010101101101

...

Machine language seems quite simple on the surface, given that there are only zeros

and ones, but its syntax is even more complex and far more intricate than Python.

So very few programmers ever write machine language. Instead we build various

translators to allow programmers to write in high-level languages like Python or

JavaScript and these translators convert the programs to machine language for

actual execution by the CPU.

Since machine language is tied to the computer hardware, machine language is not

portable across diﬀerent types of hardware. Programs written in high-level lan-

guages can be moved between diﬀerent computers by using a diﬀerent interpreter

on the new machine or recompiling the code to create a machine language version

of the program for the new machine.

These programming language translators fall into two general categories: (1) inter-

preters and (2) compilers.

An interpreter reads the source code of the program as written by the programmer,

parses the source code, and interprets the instructions on the ﬂy. Python is an

interpreter and when we are running Python interactively, we can type a line of

Python (a sentence) and Python processes it immediately and is ready for us to

type another line of Python.

Some of the lines of Python tell Python that you want it to remember some value

for later. We need to pick a name for that value to be remembered and we can use

that symbolic name to retrieve the value later. We use the term variable to refer

to the labels we use to refer to this stored data.

>>> x = 6

>>> print(x)

>>> y = x * 7

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