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`bzrlib.tests.test_import_tariff` has some tests that measure how many
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Python modules are loaded to run some representative commands.
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We want to avoid loading code unnecessarily, for reasons including:
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* Python modules are interpreted when they're loaded, either to define
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classes or modules or perhaps to initialize some structures.
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* With a cold cache we may incur blocking real disk IO for each module.
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* Some modules depend on many others.
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* Some optional modules such as `testtools` are meant to be soft
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dependencies and only needed for particular cases. If they're loaded in
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other cases then bzr may break for people who don't have those modules.
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`test_import_tarrif` allows us to check that removal of imports doesn't
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This is done by running the command in a subprocess with
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``--profile-imports``. Starting a whole Python interpreter is pretty
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slow, so we don't want exhaustive testing here, but just enough to guard
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against distinct fixed problems.
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Assertions about precisely what is loaded tend to be brittle so we instead
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make assertions that particular things aren't loaded.
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Unless selftest is run with ``--no-plugins``, modules will be loaded in
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the usual way and checks made on what they cause to be loaded. This is
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probably worth checking into, because many bzr users have at least some
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plugins installed (and they're included in binary installers).
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In theory, plugins might have a good reason to load almost anything:
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someone might write a plugin that opens a network connection or pops up a
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gui window every time you run 'bzr status'. However, it's more likely
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that the code to do these things is just being loaded accidentally. We
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might eventually need to have a way to make exceptions for particular
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Some things to check:
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* non-GUI commands shouldn't load GUI libraries
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* operations on bzr native formats sholudn't load foreign branch libraries
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* network code shouldn't be loaded for purely local operations
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* particularly expensive Python built-in modules shouldn't be loaded
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unless there is a good reason
added
removed
doc/developers/testing.txt
