Syntax and Structure

Python uses indentation instead of braces to define blocks of code. This is not a style choice, it is a hard requirement of the language.

Indentation

Whitespace matters

Mixing tabs and spaces will raise a TabError or IndentationError. Configure your editor to insert 4 spaces per tab (PEP 8 standard).

if True:
    print("this is inside the if block")
    print("still inside")
print("outside the block")

Comments

# This is a single line comment
 
"""
This is technically a multi-line string,
but when placed alone it is commonly used
as a multi-line comment or a docstring.
"""
 
x = 5  # inline comment

Statements and Line Continuation

A statement normally ends at a newline. Python has no semicolons requirement, but they can be used to separate multiple statements on one line (discouraged).

a = 1; b = 2; c = 3  # works, but avoid this in real code
 
total = 1 + 2 + 3 + \
        4 + 5  # backslash continues a line
 
total = (1 + 2 + 3 +
         4 + 5)  # parentheses are the preferred way to continue lines

The print() Function

print("Hello", "World")              # Hello World
print("Hello", "World", sep="-")     # Hello-World
print("Hello", end=" ")              # no newline after
print(f"Value is {42}")              # f-string formatting

Tip

sep and end are the two most useful keyword arguments on print() that beginners forget about.

Identifiers and Naming Rules

• Must start with a letter or underscore, not a digit.
• Can contain letters, digits, underscores.
• Case sensitive: age and Age are different variables.
• Cannot be a reserved keyword (class, for, import, etc).

_valid = True
valid_2 = True
2invalid = True   # SyntaxError

Keywords

Python reserves certain words that cannot be used as identifiers. Check the current list programmatically:

import keyword
print(keyword.kwlist)

The Python Interpreter and Execution Model

Python is interpreted (technically compiled to bytecode then run on the CPython virtual machine). Files run top to bottom at import/execution time, which is why function and class definitions must appear before they are called (unless using certain deferred patterns).

def greet():
    print("hi")
 
greet()  # must be defined above this line

The if __name__ == "__main__": Pattern

Every Python module has a built-in __name__ variable. When a file is run directly, __name__ equals "__main__". When imported, it equals the module’s name.

def main():
    print("Running as a script")
 
if __name__ == "__main__":
    main()

Why this matters

This pattern lets a file be both an importable module and a runnable script. Code inside the if block only executes when the file is run directly, not when another file imports it.

PEP 8 in One Glance

• 4 spaces per indentation level.
• Max line length around 79 to 99 characters (many teams use 88, Black’s default).
• snake_case for variables and functions, PascalCase for classes, UPPER_CASE for constants.

See PEP8-and-Style for the full breakdown.