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🐍 Python 78 guides · updated 2026

From first variable to OOP, generators, and real projects — the language that runs everything from data pipelines to AI agents, taught the practical way.

Python Syntax and Indentation: The Rules That Make Python Read Like English

The first thing developers notice when they come to Python from another language is the absence of curly braces. The second thing is usually an IndentationError. Once you understand why Python uses whitespace to define structure — and how to work with it consistently — the syntax feels natural fast.

What Syntax Actually Means

Syntax is the set of rules a language uses to determine whether a statement is valid. In Python, the rules are minimal but strict:

That last point trips up many beginners. Every if, else, elif, for, while, def, class, try, except, and with statement that introduces a new block must end with a colon before the indented code begins.

How Indentation Defines Blocks

In languages like C, Java, or JavaScript, indentation is purely cosmetic — the interpreter doesn’t care about it. In Python, indentation is structural. The interpreter uses it to determine which lines belong to which block.

x = 10


if x > 5:
    print("x is greater than five")   # inside the if block
    print("still inside")             # also inside
print("this is outside the if block") # back at the top level

Python’s official style guide (PEP 8) recommends 4 spaces per indentation level. Most editors can be configured to insert 4 spaces when you press Tab, which is the sensible setup.

Nested blocks require nested indentation

Each level of nesting adds another 4 spaces:

for i in range(3):
    print(f"Outer loop: {i}")
    for j in range(2):
        print(f"  Inner loop: {j}")    # 8 spaces in
    print("Back in outer loop")        # 4 spaces in

The visual hierarchy here directly matches the logical hierarchy of the code.

Common Indentation Errors

IndentationError: expected an indented block

This happens when Python expects a block but finds nothing:

def greet(name):
# This will fail — the function body is empty
print("Hello")

Fix it by indenting the function body:

def greet(name):
    print(f"Hello, {name}")

IndentationError: unexpected indent

This happens when a line is indented but shouldn’t be:

x = 5
    print(x)  # IndentationError — not inside any block

Mixing tabs and spaces

Python 3 refuses to run code that mixes tabs and spaces in the same block. This was a source of subtle bugs in Python 2 and the restriction exists to prevent it. Configure your editor to always use spaces, and you’ll never encounter this.

The Colon is Not Optional

Every statement that opens a new block requires a colon at the end. Forgetting it is one of the most common syntax errors for new Python developers:

# Wrong — SyntaxError
if temperature > 100
    print("Too hot")


# Correct
if temperature > 100:
    print("Too hot")

This applies to functions, classes, loops, conditionals, exception handlers, and context managers — everything that introduces an indented block.

Other Key Syntax Rules

Line continuation

Long expressions can be split across lines inside brackets, braces, or parentheses without needing anything special:

# Implicit continuation inside parentheses
total = (
    first_value
    + second_value
    + third_value
)


# Explicit continuation with backslash (use sparingly)
result = first_value + \
         second_value

The implicit form inside brackets is preferred. Backslash continuation works but is fragile — a trailing space after the backslash will silently break it.

Multiple statements on one line

Python allows semicolons to separate statements on a single line, but this is almost never good style:

x = 1; y = 2; z = 3  # works but avoid this

Comments

Single-line comments use #. There is no dedicated multi-line comment syntax — triple-quoted strings are sometimes used as a workaround, but they’re technically string literals, not comments.

# This is a comment


x = 42  # inline comments work too

A Complete Working Example

Here’s a small function that demonstrates several syntax rules together:

def classify_score(score):
    """Return a letter grade for a numeric score."""
    if score >= 90:
        grade = "A"
    elif score >= 80:
        grade = "B"
    elif score >= 70:
        grade = "C"
    elif score >= 60:
        grade = "D"
    else:
        grade = "F"


    return grade




# Call the function
for test_score in [95, 82, 67, 55]:
    result = classify_score(test_score)
    print(f"Score {test_score}: {result}")

Notice the pattern throughout: colons open blocks, indentation defines what’s inside them, and the structure is immediately readable without needing to count braces.

Tips for Getting Indentation Right

Configure your editor first. Before writing a line of Python, set your editor to use spaces (not tabs), 4 spaces per indent level, and display whitespace characters. VS Code, PyCharm, and most others can do this automatically.

Watch your copy-pasted code. Code copied from web pages sometimes brings mixed indentation with it. Running python -m py_compile yourfile.py catches syntax errors before you try to execute the program.

Trust the error message. When Python reports an IndentationError, it tells you the line number. Look at that line and the lines immediately above it — the problem is almost always there.

Python’s syntax feels constraining to developers used to free-form indentation, but it enforces consistency that makes code easier to read across teams and time. Once it becomes habit, most Python developers report they actually prefer it.