Start Coding With Python

What Is a Program?

A program is a sequence of instructions that specifies how to perform a computation. The computation might be something mathematical, such as solving a system of equat ions or finding the roots of a polynomial, but it can also be a symbolic computation, such as searching and replacing text in a document or (strangely enough) compiling a program. The details look different in different languages, but a few basic instructions appear in just about every language:

input:

  • Get data from the keyboard, a file, or some other device.

output:

  • Display data on the screen or send data to a file or other device.

math:

  • Perform basic mathematical operations like addition and multiplication.

conditional execution:

  • Check for certain conditions and execute the appropriate code.

repetition:

  • Perform some action repeatedly, usually with some variation.

Believe it or not, that’s pretty much all there is to it. Every program you’ve ever used, no matter how complicated, is made up of instructions that look pretty much like these. So you can think of programming as the process of breaking a large, complex task into smaller and smaller subtasks until the subtasks are simple enough to be performed with one of these basic instructions. That may be a little vague, but we will come back to this topic when we talk about algorithms.

Source:
Think Python by Allen B. Downey - 2012

Run Python Scripts

If you can't execute or run a Python script, then programming is pointless. When you run a Python script, the interpreter converts a Python program into something that that the computer can understand. Executing a Python program can be done in two ways: calling the Python interpreter with a shebang line, and using the interactive Python shell.

Run a Python Script as a File

Generally programmers write stand alone scripts, that are independent to live environments. Then they save it with a ".py" extension, which indicates to the operating system and programmer that the file is actually a Python program. After the interpreter is invoked, it reads and interprets the file. The way Python scripts are run on Windows versus Unix based operating systems is very different. We'll show you the difference, and how to run a Python script on Windows and Unix platforms.

Run a Python script under Windows with the Command Prompt

Windows users must pass the path of the program as an argument to the Python interpreter. Such as follows:

C:\Python27\python.exe C:\Users\Username\Desktop\my_python_script.py

Note that you must use the full path of the Python interpreter. If you want to simply type python.exe C:\Users\Username\Desktop\my_python_script.py you must add python.exe to your PATH environmental variable. Note that Windows comes with two Python executables - python.exe and pythonw.exe. If you want a terminal to pop-up when you run your script, use python.exe However if you don't want any terminal pop-up, use pythonw.exe. pythonw.exe is typically used for GUI programs, where you only want to display your program, not the terminal.

Run a Python Script Under Mac, Linux, BSD, Unix, etc

On platforms like Mac, BSD or Linux (Unix) you can put a "shebang" line as first line of the program which indicates the location of the Python interpreter on the hard drive. It's in the following format:

#!/path/to/interpreter

A common shebang line used for the Python interpreter is as follows:

#!/usr/bin/env python

You must then make the script executable, using the following command:

chmod +x my_python_script.py

Unlike Windows, the Python interpreter is typically already in the $PATH environmental variable, so adding it is un-necessary.

You can then run a program by invoking the Python interpreter manually as follows:

python firstprogram.py

Python Execution with the Shell (Live Interpreter)

Assuming that you already have Python installed and running well (if you're getting an error, see this post), open the terminal or console and type 'python' and hit the 'Enter' key. You will then be directed immediately to the Python live interpreter. Your screen will display a message something like:

user@hostname:~ python
Python 3.3.0 (default, Nov 23 2012, 10:26:01) 
[GCC 4.2.1 Compatible Apple Clang 4.1 ((tags/Apple/clang-421.11.66))] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>>

The Python programmer should keep in mind one thing: that while working with the live interpreter, everything is read and interpreted in real-time. For example loops iterate immediately, unless they are part of function. So it requires some mental planning. Using the Python shell is typically used to execute code interactively. If you want to run a Python script from the interpreter, you must either import it or call the Python executable.

Source:
http://pythoncentral.io/execute-python-script-file-shell/

A Good First Program

Type the following text into a single file named ex1.py. Python works best with files ending in .py.

print "Hello World!"
print "Hello Again"
print "I like typing this."
print "This is fun."
print 'Yay! Printing.'
print "I'd much rather you 'not'."
print 'I "said" do not touch this.'

Your Atom text editor should look something like this on all platforms:

ex1.py.

Don't worry if your editor doesn't look exactly the same, it should be close though. You may have a slightly different window header, maybe slightly different colors, and the left side of your Atom window won't say "zedshaw" but will instead show the directory you used for saving your files. All of those differences are fine.

When you create this file, keep in mind these points:

  1. I did not type the line numbers on the left. Those are printed in the book so I can talk about specific lines by saying, "See line 5..." You do not type line numbers into Python scripts.
  2. I have the print at the beginning of the line and it looks exactly the same as what I have in ex1.py. Exactly means exactly, not kind of sort of the same. Every single character has to match for it to work. Color doesn't matter, only the characters you type.

In Terminal run the file by typing:

python ex1.py

If you did it right then you should see the same output as I in the What You Should See section of this exercise. If not, you have done something wrong. No, the computer is not wrong.

What You Should See

On Mac OS X in the Terminal you should see this: osx_terminal_ex1.
On Windows in PowerShell you should see this: win_powershell_ex1.

You may see different names, before the python ex1.py command, but the important part is that you type the command and see the output is the same as mine.

If you have an error it will look like this:

$ python ex/ex1.py
  File "ex/ex1.py", line 3
    print "I like typing this.
                             ^
SyntaxError: EOL while scanning string literal

It's important that you can read these error messages because you will be making many of these mistakes. Even I make many of these mistakes. Let's look at this line by line.

  1. We ran our command in the Terminal to run the ex1.py script.
  2. Python tells us that the file ex1.py has an error on line 3.
  3. It prints this line of code for us to see it.
  4. Then it puts a ^ (caret) character to point at where the problem is. Notice the missing " (double-quote) character?
  5. Finally, it prints out a "SyntaxError" and tells us something about what might be the error. Usually these are very cryptic, but if you copy that text into a search engine, you will find someone else who's had that error and you can probably figure out how to fix it.

Source:
LEARN PYTHON THE HARD WAY by Zed A. Shaw - 2013

Debugging and Errors

What Is Debugging?

Programming is error-prone. For whimsical reasons, programming errors are called bugs and the process of tracking them down is called debugging. Three kinds of errors can occur in a program: syntax errors, runtime errors, and se mantic errors. It is useful to distinguish between them in order to track them down more quickly.

Error Types :

Syntax Errors

Python can only execute a program if the syntax is correct; otherwise, the interpreter displays an error message. Syntax refers to the structure of a program and the rules about that structure.For example, parentheses have to come in matching pairs, so (1 + 2) is legal, but 8) is a syntax error. In English readers can tolerate most syntax errors, which is why we can read the poetry of e. e. cummings without spewing error messages. Python is not so forgiving. If there is a single syntax error anywhere in your program, Python will display an error message and quit, and you will not be able to run your program. During the first few weeks of your programming career, you will probably spend a lot of time tracking down syntax errors. As you gain experience, you will make fewer errors and find them faster.

Runtime Errors

The second type of error is a runtime error, so called because the error does not appear until after the program has started running. These errors are also called exceptions because they usually indicate that something exceptional (and bad) has happened. Runtime errors are rare in the simple programs you will see in the first few chapters, so it might be a while before you encounter one.

Semantic Errors

The third type of error is the semantic error. If there is a semantic error in your program, it will run successfully in the sense that the computer will not generate any error mess ages, but it will not do the right thing. It will do something else. Specifically, it will do what you told it to do. The problem is that the program you wrote is not the program you wanted to write. The meaning of the program (its semantics) is wrong. Identifying semantic errors can be tricky because it requires you to work backward by looking at the output of the program and trying to figure out what it is doing.

Source:
Think Python by Allen B. Downey - 2012

Reserved Words

The following list shows the Python keywords. These are reserved words and you cannot use them as constants or variables or any other identifier names. All the Python keywords contain lowercase letters only.
and exec not
as finally or
assert for pass
break from print
class global raise
continue if return
def import try
del in while
elif is with
else lambda yield
except

Source:
https://www.tutorialspoint.com