Course Four - The basic structure of the Python language

The Basic Structure of Python

By ienex

Python is a versatile programming language with similarities to Perl, C, and Java, but it has its own unique features and syntax that make it simpler and more readable.

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Writing Your First Python Program

Python allows you to write programs in two primary ways: interactive programming and scripting.

1. Interactive Programming

In interactive mode, you can start the Python interpreter without passing a script file:

$ python
Python 2.4.3 (#1, Nov 11 2010, 13:34:43)
[GCC 4.1.2 20080704 (Red Hat 4.1.2-48)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>

Type the following command:

>>> print("Hello, Python!")

The output will be:

Hello, Python!

Note: In older versions of Python (like 2.x), print statements may not require parentheses.

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2. Scripting

You can also write Python programs as script files with a .py extension. Example:

# test.py
print("Hello, Python!")

Run the script from the terminal:

$ python test.py
Hello, Python!

You can also make the script executable on Unix/Linux systems:

$ chmod +x test.py
$ ./test.py
Hello, Python!

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Python Identifiers

A Python identifier is a name used for variables, functions, classes, or other objects.

• Must start with a letter (A-Z, a-z) or underscore (_)

• Can include letters, numbers (0-9), or underscores after the first character

• Case-sensitive (myVar and myvar are different)

• Cannot include special symbols such as @, $, %

Naming conventions:

• Class names start with a capital letter.

• Variables and functions start with a lowercase letter.

• Single leading underscore (_var) indicates a “protected” identifier.

• Double leading underscore (__var) indicates a special or private identifier.

• Trailing underscore (var_) avoids conflict with reserved keywords.

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Reserved Keywords

Python has reserved words that cannot be used as identifiers:

and, as, assert, break, class, continue, def, del, elif, else, except, 
finally, for, from, global, if, import, in, is, lambda, nonlocal, not, 
or, pass, raise, return, try, while, with, yield

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Indentation and Blocks

Unlike C or Java, Python does not use braces {} to define code blocks. Instead, indentation determines blocks.

Example:

if True:
    print("True")
else:
    print("False")

All lines within the same block must have the same number of spaces. Inconsistent indentation will cause errors.

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Multi-Line Statements

Use the backslash \ to split a statement across multiple lines:

total = item_one + \
        item_two + \
        item_three

Data structures like lists, dictionaries, or tuples do not require a backslash:

days = ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday']

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Strings and Quotes

Python supports multiple ways to define strings:

word = 'word'
sentence = "This is a sentence."
paragraph = """This is a paragraph
that spans multiple lines."""

Triple quotes """ """ are used for multi-line strings.

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Comments

Comments begin with # and extend to the end of the line. They are ignored by the interpreter:

# This is a comment
print("Hello, Python!")  # Inline comment

Output:

Hello, Python!

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Python’s simple syntax, clear structure, and versatile string handling make it easy for beginners to start programming while offering advanced features for more experienced developers.