In Lib/traceback.py, the public helpers print_list() and format_list() independently contain the same formatting expression, so their outputs agree only by duplication. We propose routing print_list() through format_list() so that they share a single formatting path, with no change in behavior.

Problem

format_list() returns the formatted lines:

def format_list(extracted_list):
    return StackSummary.from_list(extracted_list).format()

print_list() writes those same lines, but re-derives them with the identical expression instead of reusing format_list():

def print_list(extracted_list, file=None):
    if file is None:
        file = sys.stderr
    for item in StackSummary.from_list(extracted_list).format():
        print(item, file=file, end="")

The outputs match only because StackSummary.from_list(extracted_list).format() is duplicated in both. Any future change to how a frame list is formatted has to be applied in both places to prevent the two public helpers from silently diverging.

Solution

Have print_list() iterate format_list():

def print_list(extracted_list, file=None):
    if file is None:
        file = sys.stderr
    for item in format_list(extracted_list):
        print(item, file=file, end="")

This eliminates the possibility of drift and mirrors the delegation already used in this module; for example, print_tb() calls print_list() rather than re-deriving extract_tb(...).format().

Linked PRs

• gh-153782: Make traceback.print_list() delegate to format_list() #153783