Indentation and Code Blocks in Python

Two Tiny Details

Python does not interpret code like a human reader. A human may understand that Print probably means the same thing as print, or that a line with a few accidental spaces should still be treated as a top-level statement. Python follows exact rules instead. Capitalization is part of a name, and whitespace at the beginning of a line affects how Python interprets that line. These details can make the difference between a program that runs and one that raises an error.

Names Depend on Capitalization

Python is case-sensitive. This means that print and Print are different names because their capitalization differs. Only print is the built-in output function described in this lesson. Writing Print does not cause Python to automatically correct the capitalization or infer what the programmer meant.

print("Ready")
Print("Ready")

The first line uses the built-in output function. The second line uses the differently capitalized name Print and can result in a NameError.

Whitespace Shows Structure

Leading whitespace means spaces or tabs placed before a line begins. At the top level, statements must start at column 1, which means they must have no leading spaces or tabs. If a top-level line begins with leading whitespace, Python raises an IndentationError. Inside structured code, indentation also makes it visible which statements belong together, so changing the leading whitespace can change whether a line is treated as part of an indented block or as a top-level line.

if ready:
    print("Start")

In this example, the leading whitespace before the print line makes the line visibly belong to the indented code block introduced by the if line. The important debugging distinction is that whitespace can be meaningful structure, not decoration. At the top level, an accidental space or Tab before a statement violates Python's rule that the statement must begin at column 1.

A Whitespace Trace

Debugging Checklist

• Writing Print instead of print.

  Python is case-sensitive, so Print and print are different names. Only print is the built-in output function described here.

  Fix: Use the exact lowercase spelling print.

• Leaving spaces or a Tab before a top-level statement.

  Top-level statements must start at column 1. Leading whitespace at the top level causes an IndentationError.

  Fix: Remove all leading spaces or tabs from the beginning of the line.

• Assuming Python will infer the intended capitalization.

  Python follows exact naming rules rather than correcting a human-readable variation.

  Fix: Compare the name character by character, including capitalization.

• Checking only the visible words and ignoring the start of the line.

  Leading whitespace can determine whether a line is valid at the top level or visibly belongs to an indented block.

  Fix: Inspect the line's leading spaces and tabs whenever Python reports an IndentationError.

1. If the message is NameError, inspect the exact spelling and capitalization of the name on the reported line.
2. If the message is IndentationError, inspect the leading spaces and tabs on the reported line.
3. For a top-level statement, remove leading whitespace so the statement begins at column 1.
4. Check nearby lines for the same copied or auto-inserted whitespace problem.
5. Run the program again after correcting the specific mismatch.

Why Strict Rules Help

Case-sensitivity lets Python distinguish between related but different names, such as score and Score. Indentation makes code structure visible, so a reader can see which statements belong together. Together, these rules prevent code from having multiple possible interpretations. Python requires the programmer to state the details exactly, so the program means one clear thing rather than depending on a reader's guess.

Essential Takeaways

1. Python is case-sensitive: print and Print are different names, and only print is the built-in output function described here.
2. Leading spaces or tabs at the start of a top-level line cause an IndentationError; top-level statements must begin at column 1.
3. Indentation makes it visible which statements belong together inside structured code.
4. Use NameError messages to investigate spelling and capitalization, and IndentationError messages to investigate whitespace.
5. Strict naming and indentation rules prevent ambiguous code and make Python programs readable and unambiguous.