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?"
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 helpful to us if we only knew the language to speak to explain to the computer what we would like it to "do next". 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 assistant" quickly told us that the word "to" was used sixteen times in the first three paragraphs of this chapter.
This very fact that computers are good at things that humans are not is why you need to become skilled at talking "computer language". 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.
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:
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.
Before we start learning the language we speak to give instructions to computers to develop software, we need to learn a small amount about how computers are built. If you were to take apart your computer or cell phone and look deep inside, you would find the following parts:
The high-level definitions of these parts are as follows:
The Central Processing Unit (or CPU) is the part of the computer that is built to be obsessed with "what is next?" If your computer is rated at 3.0 Gigahertz, it means that the CPU will ask "What next?" three billion times per second. 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, microphone, speaker, touchpad, etc. 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.
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.
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.
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 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".
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.
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 words1.
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:
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.
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. >>>
>>> 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:
>>> 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 probably 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.
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.
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 ...