The pdb interact command#

Today Carlton told me about the interact command in the Python debugger.

Here’s how to use it with pytest (but it works anywhere else where you find yourself in a pdb session).

Use pytest --pdb to cause pytest to open a debugger at the first failed assertion (I added assert False to my test suite to demonstrate this).

Then type interact to drop into a full Python interactive prompt that keeps all of the local and global variables from the debugger:

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% pytest -k test_drop --pdb
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======== test session starts ========
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platform darwin -- Python 3.10.3, pytest-7.1.3, pluggy-1.0.0
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...
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>       assert False
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E       assert False
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tests/test_api_write.py:272: AssertionError
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>>>> entering PDB >>>>
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>>>> PDB post_mortem (IO-capturing turned off) >>>>
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> /Users/simon/Dropbox/Development/datasette/tests/test_api_write.py(272)test_drop_table()
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-> assert False
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(Pdb) interact
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>>> locals().keys()
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dict_keys(['__name__', '__doc__', '__package__', '__loader__', '__spec__', '__file__', '__cached__', '__builtins__',
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  '@py_builtins', '@pytest_ar', 'Datasette', 'sqlite3', 'pytest', 'time', 'ds_write', 'write_token', 'test_write_row',
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  'test_write_rows', 'test_write_row_errors', 'test_delete_row', 'test_drop_table', 'scenario', 'token', 'should_work',
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  'path', 'response', '@py_assert0', '@py_format2'])

Crucially, once you are in the interactive prompt you can inspect local variables with names like s and c without accidentally triggering matching debugger commands.

Hit Ctrl+D to exit back to the debugger:

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>>> <Ctrl+D>
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now exiting InteractiveConsole...
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(pdb)