pytest-collect-profile

Find what makes pytest collection slow.

pytest-collect-profile ranks pytest collector operations, attributes time to observed pytest hooks, and reports the complete collection time. It measures the suite pytest actually sees, including custom collectors.

Installation

python -m pip install pytest-collect-profile

Profile collection only (recommended)

pytest --collect-profile-only

Pytest collects and selects tests, the plugin prints one bounded report, and no test setup, call, or teardown runs. The usual collected-node listing is omitted.

====================== collect profile ======================
time      collector   node
1.421s    Module      tests/integration/test_api.py
0.612s    Class       tests/test_users.py::TestUsers

collector attribution
1. Module tests/integration/test_api.py
   direct fan-out: 842 children | 842 items
   1.100s | 1 call | pytest_generate_tests
   0.050s | 7 calls | pytest_pycollect_makeitem
   0.020s | 842 calls | pytest_make_parametrize_id
   0.010s | 4 calls | other observed hooks
   0.100s | nested collectors
   0.141s | outside observed hooks
2. Class tests/test_users.py::TestUsers
   direct fan-out: 12 children | 12 items
   0.200s | 1 call | pytest_generate_tests
   0.412s | outside observed hooks

collection-level hooks
   0.300s | 1 call | pytest_collection_modifyitems

Total collection: 2.340s | 854 items

Reading the report

• The slowest collector is first. The report contains at most 10 collector rows.
• Attribution is shown for the first 3 rows, with at most 3 named hooks. Additional hooks are combined as other observed hooks without losing their total time or call count.
• A hook duration covers the complete hook call, excluding time in nested hook calls. It does not time or blame individual hook implementations or plugins.
• direct fan-out counts immediate children and items, not all descendants.
• nested collectors is direct nested collector time and appears only when nonzero.
• outside observed hooks is the measured remainder after exclusive hook time and direct nested collector time are subtracted. It is not a claim about the cause.
• Collection-level hooks are reported separately. The total collection time is measured independently, so collector rows are not expected to add up to it.

Profile collection, then run tests

pytest --collect-profile

This prints the same profile before normal test execution. Add pytest's --collect-only or --co to either profiling mode when you explicitly want its native collected-node listing.

JSON for tools

Add --collect-profile-json to either profiling mode:

pytest --collect-profile-only --collect-profile-json

The plugin replaces its text report with one compact JSON v1 line. The example below is expanded only for readability; actual output is a single physical line.

{
  "schema": "pytest-collect-profile",
  "schema_version": 1,
  "profile_complete": true,
  "collectors": [
    {
      "rank": 1,
      "duration_ns": 1421000000,
      "collector_type": "Module",
      "nodeid": "tests/integration/test_api.py",
      "attribution": {
        "direct_children": 842,
        "direct_items": 842,
        "hooks": [
          {
            "name": "pytest_generate_tests",
            "duration_ns": 1100000000,
            "call_count": 1
          }
        ],
        "nested_collectors_ns": 100000000,
        "outside_observed_hooks_ns": 221000000
      }
    }
  ],
  "collection_hooks": [
    {
      "name": "pytest_collection_modifyitems",
      "duration_ns": 300000000,
      "call_count": 1
    }
  ],
  "total_collection_ns": 2340000000,
  "item_count": 854
}

The schema is deliberately bounded and complete:

• schema is always pytest-collect-profile, and schema_version is 1. profile_complete: true means the object covers the complete collection phase that ran; it does not mean collection finished without errors. An interrupted collection emits no JSON object.
• collectors contains at most 10 ranked entries. Each entry has rank, duration_ns, collector_type, nodeid, and attribution.
• attribution is present for ranks 1–3 and is null for later ranks. Its fields are direct_children, direct_items, hooks, nested_collectors_ns, and outside_observed_hooks_ns.
• Fan-out values are integers or null when unavailable. Hook lists are empty arrays when no matching hooks were observed.
• Hook entries contain name, duration_ns, and call_count. Both collector and collection-level hook lists use the same 3-plus-folded-remainder bound as the text report; other observed hooks is a synthetic aggregate entry, not an actual hook name.
• All durations, including outside_observed_hooks_ns, are unrounded integer nanoseconds. total_collection_ns is measured independently, and item_count is the selected item count at the reporting boundary.
• The root session keeps its raw empty nodeid in JSON; the text report displays it as <session>.

Pytest diagnostics and framing remain in the terminal stream, so the complete stdout is not a JSON document. Consumers should locate the line whose schema is pytest-collect-profile and parse that object. The serialized line is ASCII-only so it remains valid JSON across terminal encodings; parsing restores the original Unicode strings.

If pytest's terminal plugin is disabled, the profiler has no terminal destination and emits neither the text report nor JSON.

The modifier alone is a usage error; combine it with --collect-profile or --collect-profile-only.

For xdist projects, collect serially when consuming JSON:

pytest -n0 --collect-profile-only --collect-profile-json

Active distributed collection is not aggregated. Profile-only mode rejects it; regular profiling can otherwise produce a controller-local report with item_count: 0 and no collectors.

Compatibility and boundaries

The plugin requires Python 3.10 or newer and pytest 8.0 or newer. CI covers Python 3.10 through 3.14 on Linux, macOS, and Windows with representative pytest 8.x and 9.x releases.

Pytest continues to own selection, diagnostics, warnings, verbosity, and exit status. The plugin is silent unless a profiling mode is selected, stores no history or cache, and makes no network requests.

The project profiles collection only. It does not diagnose a root cause, profile test execution, fixtures, functions, or call stacks, or provide file exports, history, comparisons, thresholds, advice, or configurable rankings.

See the contribution guide for contribution guidance. This project is available under the terms of the MIT License.