Python Programs

Python Programs#

A Python program is simply a text file that contains Python statements.

  • All Python programs should have the extension .py

  • Only one command per line is allowed

  • The statementsin the file are interpreted line by line

To execute a Python program, you run it from the command line with:

    python my_program.py

For those unfamiliar with the command line all Python editors have shortcuts to make running a program simpler.

Variables#

Variables are ‘named containers’ used to store values within Python. Variable names may be composed of letters, underscores and, after the first position, also digits. Lowercase letters are common, uppercase letters are usually used for constants like PI.

Variables can be used for calculating in place of the values they contain.

Variable assignments#

The operator = is used in Python to define a variable. A Python statement containing an = is called a variable assignment. The value on the right side of the equal sign will be stored in a variable with the name on the left side.

Changing variables#

You may assign to the same variable name twice:

    In [1]: emily = 25952
    In [2]: emily = 222
    In [3]: emily
    Out[3]: 222

In this case, the first value is overwritten by the second assignment. There is no way to obtain it afterwards.

Python Statements#

The lines you are writing all the time are also called Python Statements. A Statement is the smallest executable piece of code.

So far, you have seen at least three different kinds of statements:

  • Calculate a number

  • Put a number into a variable

  • Print the number from a variable on the screen

Reserved words#

Some words like import, for and in are called reserved words. They have a special meaning in Python, which means that you cannont call a variable for or in. There are 33 reserved words in Python 3.

You can see the complete list with the commands:

    import keyword
    keyword.kwlist

Code blocks and indentation#

In Python, blocks of code are defined by indentation. They occur usually after a control flow statement (for, if) or a structural element (e.g. a function). A code block contains one or more line that belong to it. Python recognizes this code block by indentation, meaning that each line starts with four extra spaces. All indented code blocks start with a colon (:) at the end of the line.

Indentation is a central element of Python syntax. Indentation must not be used for decorative purposes.

Comments#

Comments are lines that are not executed. They allow you to document your code to make it easier to read. Also, you can temporarily disable lines of code. There are different ways to write comments:

    # this is a one-line comment

    """This is also a one-line comment"""

    """
    With triple quotes comments can
    stretch over multiple lines.
    """

    '''triple single quotes work like triple double quotes.'''