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MR002: branch-expression

docs/guides/lint_rules/rules/branch_expression.md

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MR002: branch-expression

⚠️ Runtime ❌ Not Fixable

MR002: Branch statements with output expressions that won't be displayed.

Why is this bad?

When output expressions are nested inside branches at the end of a cell:

  • The expressions execute but produce no visible output
  • Users expect to see the result (like mo.md(), string literals, etc.)
  • This can lead to confusion about whether code is running correctly
  • It violates the principle of least surprise

This is a runtime issue because it causes unexpected behavior where the user's intended output is silently ignored.

Examples

Problematic:

python
if condition:
    mo.md("Result A")  # Won't be displayed
else:
    mo.md("Result B")  # Won't be displayed

Problematic:

python
match value:
    case 1:
        "Too short"  # Won't be displayed
    case _:
        value  # Won't be displayed

Not flagged:

python
if condition:
    print("Debug message")  # Function calls

Solution:

python
# Assign to a variable that marimo will display
result = mo.md("Result A") if condition else mo.md("Result B")
result

Solution:

python
# Create a default variable for response.
result = None
if condition:
    result = expr
else:
    result = other
result

Alternative Solution (if no output intended):

python
# Use a dummy variable to indicate intentional suppression
if condition:
    _ = expr
else:
    _ = other

References