Python uses indentation to mark code blocks, where languages such as C and Java use braces {}. All lines in one block start at the same indentation level, and a deeper level opens a nested block. The parser reads this whitespace, so the layout is part of the syntax.
if x > 0:
print("Positive") # 4 spaces indentation
if x > 10:
print("Greater than 10") # 8 spaces indentation
else:
print("Non-positive")
It matters for two reasons. It gives every Python file the same consistent shape, which makes code easier to read. It also means a wrong indent either moves a line into a different block or stops the file from running. Incorrect indentation raises IndentationError, and mixing tabs and spaces inconsistently raises TabError. PEP 8 recommends four spaces per level.
Rewriting in plainer words…
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