sys.settrace suffers quadratic behavior for large dictionary literals on 3.12+

Bug report

Bug description:

Using a trace function with sys.settrace, performance suffers when importing a large dictionary literal. The performance exhibits quadratic time complexity on 3.12 and up. It is fine on 3.11.

This originated with a coverage.py bug report: https://github.com/nedbat/coveragepy/issues/1906

To reproduce, create make_mapping.py:

"""
Create a really large dict to import.
"""

import random

print("MAPPING = {")
for n in range(25_000):
    print(f"    '{n:08d}': {n},")
print("}")

and trace_it.py:

import linecache, sys, time

t0 = tlast = time.monotonic_ns()

def trace(frame, event, arg):
    global tlast
    if "mapping.py" in frame.f_code.co_filename:
        lineno = frame.f_lineno
        now = time.monotonic_ns()
        print("{:12d} ({}): {} {:05d}".format(
            now - t0, now - tlast,
            event[:4], lineno,
        ))
        tlast = now
    return trace

print(sys.version)
sys.settrace(trace)

import mapping

Then, create the large mapping, and run trace_it.py on multiple versions, and look at the timings:

python3 make_mapping.py > mapping.py
python3.11 trace_it.py > 11.out
python3.12 trace_it.py > 12.out
python3.13 trace_it.py > 13.out
python3.14 trace_it.py > 14.out
grep 'line \d[05]000' *.out

These are the results I see. The parenthetical numbers are nanoseconds since the last output line:

11.out:    76131833 (1917): line 05000
11.out:    86116875 (917): line 10000
11.out:    96115250 (917): line 15000
11.out:   106099958 (958): line 20000
11.out:   116161416 (875): line 25000

12.out:   803728958 (148833): line 05000
12.out:  3009835500 (293042): line 10000
12.out:  6750690750 (444833): line 15000
12.out: 11966781625 (596292): line 20000
12.out: 18720473667 (739000): line 25000

13.out:   443250333 (75125): line 05000
13.out:  1565451416 (147583): line 10000
13.out:  3447408625 (220625): line 15000
13.out:  6130466958 (294458): line 20000
13.out:  9455595416 (370500): line 25000

14.out:   445441625 (89291): line 05000
14.out:  1556189584 (152250): line 10000
14.out:  3410512125 (220625): line 15000
14.out:  6007919334 (366584): line 20000
14.out:  9369414792 (375083): line 25000

CPython versions tested on:

3.11, 3.12, 3.13, 3.14

Operating systems tested on:

macOS

Linked PRs

• gh-128350
• gh-129127
• gh-132268

Start by running make_mapping.py and trace_it.py with sys.settrace across the listed Python versions, then compare the timing output at each 5,000-line interval. Review linked PRs gh-128350, gh-129127, and gh-132268 before investigating further; done means large dictionary imports no longer show quadratic timing under tracing.