Text stringlengths 1 9.41k |
|---|
All advertising materials mentioning features or use of this software
* must display the following acknowledgement:
* "This product includes cryptographic software written by
* Eric Young (eay@cryptsoft.com)"
* The word 'cryptographic' can be left out if the rouines from the library
* being used are not crypt... |
If you include any Windows specific code (or a derivative thereof) from
* the apps directory (application code) you must include an acknowledgement:
* "This product includes software written by Tim Hudson (tjh@cryptsoft.com)"
*
* THIS SOFTWARE IS PROVIDED BY ERIC YOUNG ``AS IS'' AND
* ANY EXPRESS OR IMPLIED W... |
IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
* HOWEVER CAUSED AND ON ANY THEORY... |
i.e. |
this code cannot simply be
* copied and put under another distribution licence
* [including the GNU Public Licence.]
*/
#### C.3.13 expat
```
The pyexpat extension is built using an included copy of the expat sources unless the build is configured
```
--with-system-expat:
Copyright (c) 1998, 1999, 2000 Thai Ope... |
IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
DEALINGS IN THE SOFTWARE.
#### C.3.15 zlib
```
The zlib extension is built using an i... |
In no event will the authors be held liable for any damages
arising from the use of this software.
Permission is granted to anyone to use this software for any purpose,
including commercial applications, and to alter it and redistribute it
freely, subject to the following restrictions:
```
(continues on next page)
-... |
The origin of this software must not be misrepresented; you must not
claim that you wrote the original software. |
If you use this software
in a product, an acknowledgment in the product documentation would be
appreciated but is not required.
2. |
Altered source versions must be plainly marked as such, and must not be
misrepresented as being the original software.
3. |
This notice may not be removed or altered from any source distribution.
Jean-loup Gailly Mark Adler
jloup@gzip.org madler@alumni.caltech.edu
#### C.3.16 cfuhash
```
The implementation of the hash table used by the tracemalloc is based on the cfuhash project:
-----
#### C.3.17 libmpdec
The _decimal module is built... |
### Python for Everybody
## Chapter 1
# Why should you learn to write programs?
Writing programs (or programming) is a very creative and rewarding activity. |
You
can write programs for many reasons, ranging from making your living to solving
a difficult data analysis problem to having fun to helping someone else solve a
problem. |
This book assumes that everyone needs to know how to program, and
that once you know how to program you will figure out what you want to do with
your newfound skills.
We are surrounded in our daily lives with computers ranging from laptops to cell
phones. |
We can think of these computers as our “personal assistants” who can take
care of many things on our behalf. |
The hardware in our current-day computers is
essentially built to continuously ask us the question, “What would you like me to
do next?”
What What What
Next? Next? Next?
What What What
Next? Next? |
Next?
Figure 1.1: Personal Digital Assistant
Programmers add an operating system and a set of applications to the hardware
and we end up with a Personal Digital Assistant that is quite helpful and capable
of helping us do many different things.
Our computers are fast and have vast amounts of memory and could be very... |
If we knew this language, we could tell the computer
to do tasks on our behalf that were repetitive. |
Interestingly, the kinds of things
computers can do best are often the kinds of things that we humans find boring
and mind-numbing.
-----
For example, look at the first three paragraphs of this chapter and tell me the
most commonly used word and how many times the word is used. |
While you were
able to read and understand the words in a few seconds, counting them is almost
painful because it is not the kind of problem that human minds are designed to
solve. |
For a computer the opposite is true, reading and understanding text from
a piece of paper is hard for a computer to do but counting the words and telling
you how many times the most used word was used is very easy for the computer:
python words.py
Enter file:words.txt
to 16
Our “personal information analysis assistan... |
Once you learn this new
language, you can delegate mundane tasks to your partner (the computer), leaving
more time for you to do the things that you are uniquely suited for. |
You bring
creativity, intuition, and inventiveness to this partnership.
##### 1.1 Creativity and motivation
While this book is not intended for professional programmers, professional programming can be a very rewarding job both financially and personally. |
Building
useful, elegant, and clever programs for others to use is a very creative activity.
Your computer or Personal Digital Assistant (PDA) usually contains many different programs from many different groups of programmers, each competing for
your attention and interest. |
They try their best to meet your needs and give you a
great user experience in the process. |
In some situations, when you choose a piece
of software, the programmers are directly compensated because of your choice.
If we think of programs as the creative output of groups of programmers, perhaps
the following figure is a more sensible version of our PDA:
Pick Pick Pick
Me! |
Me! Me!
Pick Pick Buy
Me! Me! |
Me :)
Figure 1.2: Programmers Talking to You
For now, our primary motivation is not to make money or please end users, but
instead for us to be more productive in handling the data and information that we
will encounter in our lives. |
When you first start, you will be both the programmer
and the end user of your programs. |
As you gain skill as a programmer and programming feels more creative to you, your thoughts may turn toward developing
programs for others.
-----
##### 1.2 Computer hardware architecture
Before we start learning the language we speak to give instructions to computers
to develop software, we need to learn a small am... |
If you were to take apart your computer or cell phone and look deep inside,
you would find the following parts:
What
Software Next?
Input and Central
Output Processing Network
Devices Unit
Main Secondary
Memory Memory
Figure 1.3: Hardware Archicture
The high-level definitions of these parts are as follows:
- T... |
You are going to have to learn how to talk fast to keep up with
the CPU.
- The Main Memory is used to store information that the CPU needs in a
hurry. The main memory is nearly as fast as the CPU. |
But the information
stored in the main memory vanishes when the computer is turned off.
- The Secondary Memory is also used to store information, but it is much
slower than the main memory. |
The advantage of the secondary memory is
that it can store information even when there is no power to the computer.
Examples of secondary memory are disk drives or flash memory (typically
found in USB sticks and portable music players).
- The Input and Output Devices are simply our screen, keyboard, mouse, microphon... |
They are all of the ways we interact with
the computer.
- These days, most computers also have a Network Connection to retrieve
information over a network. |
We can think of the network as a very slow
place to store and retrieve data that might not always be “up”. |
So in a sense,
the network is a slower and at times unreliable form of Secondary Memory.
While most of the detail of how these components work is best left to computer
builders, it helps to have some terminology so we can talk about these different
parts as we write our programs.
Secondary
Memory
-----
As a progr... |
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.
What
Software Next?
Input and Central
Output Processing Network
Devices Unit
Main
Memory Secondary
Memory
Figure 1.4: Where Are You?
You need to be the person who answers the CPU’s “What next?” questio... |
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 person 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 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
Secondary
-----
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 increasingly 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 use 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 reserved 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[1].
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 fro... |
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 b... |
Our
sentence starts with the function print followed by a string of text of our choosing
enclosed in single quotes.
##### 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 yo... |
That is too much
[detail for this chapter so I suggest that you consult www.pythonlearn.com where](http://www.pythonlearn.com)
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 "... |
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 g... |
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:
>>> print('Hello world!')
Hello world... |
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... |
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 probably know the proper way to say “good-bye” when interacting with the inhabitants
of Planet Python:
>>> good-bye
Tr... |
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.
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 humans to read and write and for computers to read and pr... |
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 understand 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 a... |
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 differen... |
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) interpre... |
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)
6
>>> y = x * 7
>>> print(y)
42
>>>
In this example, we ask Python to remember the value six and use the label x so
we can retrieve the value later. |
We verify that Python has actually remembered
-----
the value using print. Then we ask Python to retrieve x and multiply it by seven
and put the newly computed value in y. |
Then we ask Python to print out the
value currently in y.
Even though we are typing these commands into Python one line at a time, Python
is treating them as an ordered sequence of statements with later statements able
to retrieve data created in earlier statements. |
We are writing our first simple
paragraph with four sentences in a logical and meaningful order.
It is the nature of an interpreter to be able to have an interactive conversation
as shown above. |
A compiler needs to be handed the entire program in a file,
and then it runs a process to translate the high-level source code into machine
language and then the compiler puts the resulting machine language into a file for
later execution.
If you have a Windows system, often these executable machine language programs
... |
In Linux and Macintosh, there is no suffix that uniquely
marks a file as executable.
If you were to open an executable file in a text editor, it would look completely
crazy and be unreadable:
^?ELF^A^A^A^@^@^@^@^@^@^@^@^@^B^@^C^@^A^@^@^@\xa0\x82
^D^H4^@^@^@\x90^]^@^@^@^@^@^@4^@ ^@^G^@(^@$^@!^@^F^@
^@^@4^@^@^@4\x80^D^... |
What language is it written
in? Is it written in a compiled language? When we type “python”, what exactly
is happening?
The Python interpreter is written in a high-level language called “C”. |
You can look
[at the actual source code for the Python interpreter by going to www.python.org](http://www.python.org)
and working your way to their source code. |
So Python is a program itself and it
is compiled into machine code. |
When you installed Python on your computer (or
the vendor installed it), you copied a machine-code copy of the translated Python
program onto your system. |
In Windows, the executable machine code for Python
itself is likely in a file with a name like:
C:\Python35\python.exe
That is more than you really need to know to be a Python programmer, but
sometimes it pays to answer those little nagging questions right at the beginning.
-----
##### 1.7 Writing a program
Typin... |
By convention, Python scripts
have names that end with .py.
To execute the script, you have to tell the Python interpreter the name of the file.
In a Unix or Windows command window, you would type python hello.py as
follows:
**csev$ cat hello.py**
**print('Hello world!')**
**csev$ python hello.py**
**Hello world!**
*... |
When Python is reading your source code from a file, it knows
to stop when it reaches the end of the file.
##### 1.8 What is a program?
The definition of a program at its most basic is a sequence of Python statements
that have been crafted to do something. |
Even our simple hello.py script is a
program. |
It is a one-line program and is not particularly useful, but in the strictest
definition, it is a Python program.
It might be easiest to understand what a program is by thinking about a problem
that a program might be built to solve, and then looking at a program that would
solve that problem.
Lets say you are doing ... |
You could print out
the stream of Facebook posts and pore over the text looking for the most common
word, but that would take a long time and be very mistake prone. |
You would be
smart to write a Python program to handle the task quickly and accurately so you
can spend the weekend doing something fun.
For example, look at the following text about a clown and a car. |
Look at the text
and figure out the most common word and how many times it occurs.
the clown ran after the car and the car ran into the tent
and the tent fell down on the clown and the car
-----
Then imagine that you are doing this task looking at millions of lines of text.
Frankly it would be quicker for you to le... |
I wrote it, tested it, and now I am giving it to
you to use so you can save some time.
name = input('Enter file:')
handle = open(name, 'r')
text = handle.read()
words = text.split()
counts = dict()
**for word in words:**
counts[word] = counts.get(word, 0) + 1
bigcount = None
bigword = None
**for word, count in list(c... |
You will need to
get through Chapter 10 of this book to fully understand the awesome Python
techniques that were used to make the program. |
You are the end user, you simply
use the program and marvel at its cleverness and how it saved you so much manual
effort. |
You simply type the code into a file called words.py and run it or you
[download the source code from http://www.pythonlearn.com/code3/ and run it.](http://www.pythonlearn.com/code3/)
This is a good example of how Python and the Python language are acting as an
intermediary between you (the end user) and me (the progr... |
Python is a
way for us to exchange useful instruction sequences (i.e., programs) in a common
language that can be used by anyone who installs Python on their computer. |
So
neither of us are talking to Python, instead we are communicating with each other
_through Python._
##### 1.9 The building blocks of programs
In the next few chapters, we will learn more about the vocabulary, sentence structure, paragraph structure, and story structure of Python. |
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