What happens when the if statement is not indented?

When an if statement in Python is not properly indented, the Python interpreter will raise an IndentationError or SyntaxError, preventing the program from running.

Python is unique among many programming languages because it uses indentation to define code blocks, rather than relying on curly braces {} or keywords like begin/end. This design choice makes Python code exceptionally readable but also makes indentation a critical part of its syntax.

The Critical Role of Indentation in Python

Indentation in Python isn't just a stylistic preference; it's a fundamental part of the language's syntax. Python treats indentation much like other languages treat braces, using it to determine the scope of code blocks associated with control structures such as:

if/elif/else statements
for and while loops
Function definitions (def)
Class definitions (class)
try/except blocks
with statements

Without correct indentation, the Python interpreter cannot logically understand which lines of code belong to which block. This leads to fatal errors during the parsing or execution phase, as the interpreter cannot correctly delineate the structure of your program.

What Happens Without Indentation?

Let's break down the consequences:

IndentationError: This is the most common error you'll encounter. Python expects a new block of code following a statement like if condition:. If the line immediately after this colon is not indented, or if subsequent lines within the intended block have inconsistent indentation, an IndentationError will be raised. This error occurs because the interpreter explicitly expects an indented block.
SyntaxError: In some less common scenarios, particularly if the lack of indentation is part of a broader structural issue, Python might raise a SyntaxError. This indicates that the interpreter cannot parse the code according to Python's grammatical rules.
Program Halts: Regardless of the specific error type, the program will not run. The interpreter stops processing the script as soon as it encounters the indentation issue.

Example: Incorrect Indentation

Consider this simple Python code snippet:

x = 10
if x > 5:
print("x is greater than 5") # Incorrectly not indented
print("This line is outside the if block")

When you try to execute this code, Python will immediately report an error:

  File "<stdin>", line 3
    print("x is greater than 5") # Incorrectly not indented
    ^
IndentationError: expected an indented block

The IndentationError clearly states that an indented block was expected after the if x > 5: line.

Example: Correct Indentation

Here's how the same code should be written with proper indentation:

x = 10
if x > 5:
    print("x is greater than 5") # Correctly indented by 4 spaces
print("This line is outside the if block")

In this correct version, the line print("x is greater than 5") is indented, making it part of the if block. The line print("This line is outside the if block") is not indented at the same level as the if block, correctly signifying that it will execute regardless of the if condition.

Why Indentation is So Important

Aspect	Non-Indented `if` Statement (Incorrect)	Indented `if` Statement (Correct)
Interpreter's View	Cannot determine the block's start and end points.	Clearly identifies which code belongs to the `if` condition.
Error Type	`IndentationError` or `SyntaxError`.	No indentation-related error.
Execution	Program halts immediately; no code runs.	Program executes as intended, following conditional logic.
Readability	Difficult to understand code structure at a glance.	Enhances code readability and maintainability significantly.
Impact	Prevents the entire script from functioning.	Ensures logical flow and correct program behavior.

Best Practices for Indentation

To avoid these errors and write robust Python code, follow these guidelines:

Consistency: Always use the same method for indentation throughout your project. Python's official style guide, PEP 8, recommends using 4 spaces for each level of indentation.
Avoid Tabs and Spaces Mix: Mixing tabs and spaces for indentation is a common source of IndentationError because they are treated differently by the interpreter, even if they appear visually identical in some editors. Most modern code editors convert tabs to spaces automatically or warn you about mixed indentation.
Utilize IDEs/Editors: Integrated Development Environments (IDEs) like PyCharm, VS Code, or Sublime Text automatically handle indentation for you, making it much easier to write syntactically correct Python.
Linters: Tools like flake8 or Pylint can analyze your code for style guide violations, including inconsistent or incorrect indentation, even before you run it.

In summary, proper indentation is not merely a convention but a core syntax requirement in Python. Failing to indent an if statement's block will invariably lead to an IndentationError, making your code unexecutable.