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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: nedbat/coveragepy#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 mappingThen, 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
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only security fixesonly security fixesbugs and security fixesbugs and security fixesbugs and security fixesbugs and security fixes(Objects, Python, Grammar, and Parser dirs)(Objects, Python, Grammar, and Parser dirs)An unexpected behavior, bug, or errorAn unexpected behavior, bug, or error