doc/faq.rst - third_party/github.com/nedbat/coveragepy - Git at Google

 .. Licensed under the Apache License: http://www.apache.org/licenses/LICENSE-2.0
 .. For details: https://github.com/nedbat/coveragepy/blob/master/NOTICE.txt

 .. _faq:

 ==================
 FAQ and other help
 ==================


 Frequently asked questions
 --------------------------

 Q: Why are some of my files not measured?
 .........................................

 Coverage.py has a number of mechanisms for deciding which files to measure and
 which to skip.  If your files aren't being measured, use the ``--debug=trace``
 :ref:`option <cmd_run_debug>`, also settable as ``[run] debug=trace`` in the
 :ref:`settings file <config_run_debug>`, or as ``COVERAGE_DEBUG=trace`` in an
 environment variable.

 This will write a line for each file considered, indicating whether it is
 traced or not, and if not, why not.  Be careful though: the output might be
 swallowed by your test runner.  If so, a ``COVERAGE_DEBUG_FILE=/tmp/cov.out``
 environment variable can direct the output to a file instead to ensure you see
 everything.


 Q: Why do unexecutable lines show up as executed?
 .................................................

 Usually this is because you've updated your code and run coverage.py on it
 again without erasing the old data.  Coverage.py records line numbers executed,
 so the old data may have recorded a line number which has since moved, causing
 coverage.py to claim a line has been executed which cannot be.

 If old data is persisting, you can use an explicit ``coverage erase`` command
 to clean out the old data.


 Q: Why are my function definitions marked as run when I haven't tested them?
 ............................................................................

 The ``def`` and ``class`` lines in your Python file are executed when the file
 is imported.  Those are the lines that define your functions and classes.  They
 run even if you never call the functions. It's the body of the functions that
 will be marked as not executed if you don't test them, not the ``def`` lines.

 This can mean that your code has a moderate coverage total even if no tests
 have been written or run.  This might seem surprising, but it is accurate: the
 ``def`` lines have actually been run.


 Q: Why do the bodies of functions show as executed, but the def lines do not?
 .............................................................................

 If this happens, it's because coverage.py has started after the functions are
 defined.  The definition lines are executed without coverage measurement, then
 coverage.py is started, then the function is called.  This means the body is
 measured, but the definition of the function itself is not.

 The same thing can happen with the bodies of classes.

 To fix this, start coverage.py earlier.  If you use the :ref:`command line
 <cmd>` to run your program with coverage.py, then your entire program will be
 monitored.  If you are using the :ref:`API <api>`, you need to call
 coverage.start() before importing the modules that define your functions.


 Q: My decorator lines are marked as covered, but the "def" line is not.  Why?
 .............................................................................

 Different versions of Python report execution on different lines.  Coverage.py
 adapts its behavior to the version of Python being used.  In Python 3.7 and
 earlier, a decorated function definition only reported the decorator as
 executed. In Python 3.8 and later, both the decorator and the "def" are
 reported.  If you collect execution data on Python 3.7, and then run coverage
 reports on Python 3.8, there will be a discrepancy.


 Q: Can I find out which tests ran which lines?
 ..............................................

 Yes! Coverage.py has a feature called :ref:`dynamic_contexts` which can collect
 this information.  Add this to your .coveragerc file:

 .. code-block:: ini

     [run]
     dynamic_context = test_function

 and then use the ``--contexts`` option when generating an HTML report.


 Q: How is the total percentage calculated?
 ..........................................

 Coverage.py counts the total number of possible executions. This is the number
 of executable statements minus the number of excluded statements.  It then
 counts the number of those possibilities that were actually executed.  The
 total percentage is the actual executions divided by the possible executions.

 As an example, a coverage report with 1514 statements and 901 missed
 statements would calculate a total percentage of (1514-901)/1514, or 40.49%.

 :ref:`Branch coverage <branch>` extends the calculation to include the total
 number of possible branch exits, and the number of those taken.  In this case
 the specific numbers shown in coverage reports don't calculate out to the
 percentage shown, because the number of missing branch exits isn't reported
 explicitly.  A branch line that wasn't executed at all is counted once as a
 missing statement in the report, instead of as two missing branches.  Reports
 show the number of partial branches, which is the lines that were executed but
 did not execute all of their exits.


 Q: Coverage.py is much slower than I remember, what's going on?
 ...............................................................

 Make sure you are using the C trace function.  Coverage.py provides two
 implementations of the trace function.  The C implementation runs much faster.
 To see what you are running, use ``coverage debug sys``.  The output contains
 details of the environment, including a line that says either
 ``CTracer: available`` or ``CTracer: unavailable``.  If it says unavailable,
 then you are using the slow Python implementation.

 Try re-installing coverage.py to see what happened and if you get the CTracer
 as you should.


 Q: Isn't coverage testing the best thing ever?
 ..............................................

 It's good, but `it isn't perfect`__.

 __ https://nedbatchelder.com/blog/200710/flaws_in_coverage_measurement.html


 Q: Where can I get more help with coverage.py?
 ..............................................

 You can discuss coverage.py or get help using it on the `Python discussion
 forums`_.  If you ping me (``@nedbat``), there's a higher chance I'll see the
 post.

 .. _Python discussion forums: https://discuss.python.org/

 Bug reports are gladly accepted at the `GitHub issue tracker`_.

 .. _GitHub issue tracker: https://github.com/nedbat/coveragepy/issues

 `I can be reached`__ in a number of ways. I'm happy to answer questions about
 using coverage.py.

 __  https://nedbatchelder.com/site/aboutned.html


 History
 -------

 Coverage.py was originally written by `Gareth Rees`_.
 Since 2004, `Ned Batchelder`_ has extended and maintained it with the help of
 `many others`_.  The :ref:`change history <changes>` has all the details.

 .. _Gareth Rees: http://garethrees.org/
 .. _Ned Batchelder: https://nedbatchelder.com
 .. _many others: https://github.com/nedbat/coveragepy/blob/master/CONTRIBUTORS.txt
	.. Licensed under the Apache License: http://www.apache.org/licenses/LICENSE-2.0
	.. For details: https://github.com/nedbat/coveragepy/blob/master/NOTICE.txt

	.. _faq:

	==================
	FAQ and other help
	==================


	Frequently asked questions
	--------------------------

	Q: Why are some of my files not measured?
	.........................................

	Coverage.py has a number of mechanisms for deciding which files to measure and
	which to skip. If your files aren't being measured, use the ``--debug=trace``
	:ref:`option <cmd_run_debug>`, also settable as ``[run] debug=trace`` in the
	:ref:`settings file <config_run_debug>`, or as ``COVERAGE_DEBUG=trace`` in an
	environment variable.

	This will write a line for each file considered, indicating whether it is
	traced or not, and if not, why not. Be careful though: the output might be
	swallowed by your test runner. If so, a ``COVERAGE_DEBUG_FILE=/tmp/cov.out``
	environment variable can direct the output to a file instead to ensure you see
	everything.


	Q: Why do unexecutable lines show up as executed?
	.................................................

	Usually this is because you've updated your code and run coverage.py on it
	again without erasing the old data. Coverage.py records line numbers executed,
	so the old data may have recorded a line number which has since moved, causing
	coverage.py to claim a line has been executed which cannot be.

	If old data is persisting, you can use an explicit ``coverage erase`` command
	to clean out the old data.


	Q: Why are my function definitions marked as run when I haven't tested them?
	............................................................................

	The ``def`` and ``class`` lines in your Python file are executed when the file
	is imported. Those are the lines that define your functions and classes. They
	run even if you never call the functions. It's the body of the functions that
	will be marked as not executed if you don't test them, not the ``def`` lines.

	This can mean that your code has a moderate coverage total even if no tests
	have been written or run. This might seem surprising, but it is accurate: the
	``def`` lines have actually been run.


	Q: Why do the bodies of functions show as executed, but the def lines do not?
	.............................................................................

	If this happens, it's because coverage.py has started after the functions are
	defined. The definition lines are executed without coverage measurement, then
	coverage.py is started, then the function is called. This means the body is
	measured, but the definition of the function itself is not.

	The same thing can happen with the bodies of classes.

	To fix this, start coverage.py earlier. If you use the :ref:`command line
	<cmd>` to run your program with coverage.py, then your entire program will be
	monitored. If you are using the :ref:`API <api>`, you need to call
	coverage.start() before importing the modules that define your functions.


	Q: My decorator lines are marked as covered, but the "def" line is not. Why?
	.............................................................................

	Different versions of Python report execution on different lines. Coverage.py
	adapts its behavior to the version of Python being used. In Python 3.7 and
	earlier, a decorated function definition only reported the decorator as
	executed. In Python 3.8 and later, both the decorator and the "def" are
	reported. If you collect execution data on Python 3.7, and then run coverage
	reports on Python 3.8, there will be a discrepancy.


	Q: Can I find out which tests ran which lines?
	..............................................

	Yes! Coverage.py has a feature called :ref:`dynamic_contexts` which can collect
	this information. Add this to your .coveragerc file:

	.. code-block:: ini

	[run]
	dynamic_context = test_function

	and then use the ``--contexts`` option when generating an HTML report.


	Q: How is the total percentage calculated?
	..........................................

	Coverage.py counts the total number of possible executions. This is the number
	of executable statements minus the number of excluded statements. It then
	counts the number of those possibilities that were actually executed. The
	total percentage is the actual executions divided by the possible executions.

	As an example, a coverage report with 1514 statements and 901 missed
	statements would calculate a total percentage of (1514-901)/1514, or 40.49%.

	:ref:`Branch coverage <branch>` extends the calculation to include the total
	number of possible branch exits, and the number of those taken. In this case
	the specific numbers shown in coverage reports don't calculate out to the
	percentage shown, because the number of missing branch exits isn't reported
	explicitly. A branch line that wasn't executed at all is counted once as a
	missing statement in the report, instead of as two missing branches. Reports
	show the number of partial branches, which is the lines that were executed but
	did not execute all of their exits.


	Q: Coverage.py is much slower than I remember, what's going on?
	...............................................................

	Make sure you are using the C trace function. Coverage.py provides two
	implementations of the trace function. The C implementation runs much faster.
	To see what you are running, use ``coverage debug sys``. The output contains
	details of the environment, including a line that says either
	``CTracer: available`` or ``CTracer: unavailable``. If it says unavailable,
	then you are using the slow Python implementation.

	Try re-installing coverage.py to see what happened and if you get the CTracer
	as you should.


	Q: Isn't coverage testing the best thing ever?
	..............................................

	It's good, but `it isn't perfect`__.

	__ https://nedbatchelder.com/blog/200710/flaws_in_coverage_measurement.html


	Q: Where can I get more help with coverage.py?
	..............................................

	You can discuss coverage.py or get help using it on the `Python discussion
	forums`_. If you ping me (``@nedbat``), there's a higher chance I'll see the
	post.

	.. _Python discussion forums: https://discuss.python.org/

	Bug reports are gladly accepted at the `GitHub issue tracker`_.

	.. _GitHub issue tracker: https://github.com/nedbat/coveragepy/issues

	`I can be reached`__ in a number of ways. I'm happy to answer questions about
	using coverage.py.

	__ https://nedbatchelder.com/site/aboutned.html


	History
	-------

	Coverage.py was originally written by `Gareth Rees`_.
	Since 2004, `Ned Batchelder`_ has extended and maintained it with the help of
	`many others`_. The :ref:`change history <changes>` has all the details.

	.. _Gareth Rees: http://garethrees.org/
	.. _Ned Batchelder: https://nedbatchelder.com
	.. _many others: https://github.com/nedbat/coveragepy/blob/master/CONTRIBUTORS.txt