IndentationError: expected an indented block

What this error means

A block header — if, for, while, def, class, try, else — ended with a colon, and Python then expected at least one indented statement belonging to that block. It found none.

This is a compile-time error, so nothing in the file ran. Python parses the whole file into bytecode before executing a single line.

Traceback (most recent call last):
  File "demo.py", line 4
    print("ready")
    ^
IndentationError: expected an indented block after 'if' statement on line 3

Python 3.10+ names the exact header, which makes this one of the easier messages to act on: it tells you both the line the body was missing under and the line where the parser gave up.

The four causes

# 1 — the body is simply missing
if ready:
print("ready")            # ❌ not indented

if ready:
    print("ready")        # ✅

# 2 — the body was deleted while editing
def main():
                          # ❌ nothing left under the header

# 3 — a comment is not a statement
for x in items:
    # TODO: handle x      # ❌ a comment does not count as a body
    pass                  # ✅ pass does

# 4 — the body is on the header line but a colon is missing
if ready print("ready")   # ❌ SyntaxError, not IndentationError
if ready: print("ready")  # ✅ legal, but write the block form

Cause 3 surprises people: Python counts *statements*, and a comment is not one. A block containing only comments is still an empty block.

The fix: pass

pass is a statement that does nothing. It exists precisely to fill a block that is intentionally empty:

def handler(event):
    pass                    # ✅ intentionally empty for now

class Placeholder:
    ...                     # ✅ the ellipsis literal works too

for x in items:
    if x < 0:
        pass                # ✅ skip negatives, handle the rest below
    process(x)

try:
    risky()
except ValueError:
    pass                    # ✅ swallow deliberately — but say why

pass and ... are interchangeable here. ... reads better in stub code and type definitions; pass is the conventional choice inside function bodies. Neither does anything at runtime.

One warning about except: pass. Silently swallowing an exception hides real bugs, and you will debug it later without any clue where it started. If you must ignore it, name the exception and write a comment saying why.

One-liner versus block form

Python allows a single statement on the same line as the header. It is legal, and it is also a common source of this error when someone half-converts between the two forms:

if ready: print("ready")     # ✅ one statement, same line

if ready:                    # ❌ the colon promises a block that never comes

if ready:
    print("ready")           # ✅ the form to prefer

Prefer the block form. The one-liner saves a line and costs you the ability to add a second statement later without restructuring — and it is the form most style guides discourage.

Tabs versus spaces

A closely related failure, with a different message. If the block is indented but the indentation is inconsistent, you get TabError instead:

def f():
	if x:            # tab-indented
        pass       # space-indented — ❌ TabError: inconsistent use of tabs

# and this produces IndentationError even though it looks aligned:
def f():
  if x:            # 2 spaces
      pass         # 4 spaces — the block is deeper, which is fine,
  return x         # but mixing levels across the file confuses readers

The rule is simple: pick one style and use it everywhere. PEP 8 specifies four spaces per level and no tabs. Every editor can be told to insert spaces when you press Tab, and to convert an existing file in one command.

1. In VS Code: bottom status bar shows the current indentation; click it and choose *Convert Indentation to Spaces*.
2. In Vim: :set expandtab then :retab.
3. From the command line: python -m tabnanny yourfile.py reports every mixed line with its number.

Related messages worth knowing

• IndentationError: the parent guide — the whole family of indentation failures and how Python decides blocks.
• IndentationError: unexpected indent — the mirror image — a line indented when no block was opened.
• SyntaxError: unexpected EOF while parsing — what you get on Python 3.9 and earlier when the empty block is at the end of the file.