Improve List Extension

python.improve_list_extension

Severity

low

Remediation Complexity

trivial

Remediation Risk

low

Remediation Effort

low

Resource

Efficiency

Language

Python

Description

Reports for loops whose only statement is dest.append(item) where item is the loop variable. The loop is the verbose form of dest.extend(source); the idiomatic form is shorter, runs in C, and reads more clearly.

for item in source:
    dest.append(item)              # FLAW

dest.extend(source)                # OK
dest += list(source)               # OK

Rationale

extend is implemented in C and resizes the destination once based on the source’s length (when known). The Python-level for plus append does the same logical work with one bytecode interpretation per element — measurable on large iterables and a clearer signal of intent on small ones.

Remediation

dest.extend(source)
# or, when accumulating from multiple sources:
dest.extend(a)
dest.extend(b)
# or use a comprehension to filter / transform:
dest.extend(item for item in source if predicate(item))

References

Python — list.extend