Python 3：面向大众的数据探索教程

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《Python for Everybody: Exploring Data in Python 3》是一本专为初学者设计的书籍，由Charles Severance撰写，旨在通过探索数据驱动的视角引导读者进入编程和软件开发的世界。Python在这本书中被定位为一种强大的工具，能够解决传统电子表格无法处理的复杂数据问题。该书在2016年7月首次发布，基于Python 3.0版本，是对原作《Think Python: How to Think Like a Computer Scientist》的改编和扩展。书中强调了Python作为一种通用语言的实用性，尤其在数据科学和数据库领域的应用。作者Charles Severance在编写过程中，采取了“remixing”的创新方式，即并非从零开始创作，而是对 Allen B. Downey、Jeff Elkner 等人编写的《Think Python》进行了改造，融入了自己的见解和针对Python 3的新内容。这种实践反映了现代教育中开源教材的灵活运用，鼓励学术界在现有资源基础上进行创新。本书的结构可能包括基础知识的介绍，如变量、数据类型、控制流、函数等，然后逐步深入到数据处理和分析，可能会涉及列表、字典、集合等数据结构，以及Pandas、NumPy等Python数据分析库的使用。此外，书中可能还会涉及SQL查询与数据库交互，以及如何用Python进行网络编程，以适应网络化编程的需求。在版权方面，该书遵循Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License，这意味着读者可以在非商业用途下自由分享和修改内容，但必须保留原始作者和许可信息。附录部分详细列出了关于商业使用和版权豁免的规定，体现了开放教育资源的共享精神。对于那些希望通过学习Python来解决实际数据问题的学生、教师或专业人士来说，《Python for Everybody》是一本理想的入门指南，它不仅教授编程技能，还提供了如何在现实世界中应用这些技能去探索和理解数据的方法。通过阅读这本书，读者不仅能掌握Python的基础知识，还能培养出数据驱动决策的能力。

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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