"trace" --- Trace or track Python statement execution
*****************************************************

**Source code:** Lib/trace.py

======================================================================

The "trace" module allows you to trace program execution, generate
annotated statement coverage listings, print caller/callee
relationships and list functions executed during a program run.  It
can be used in another program or from the command line.

参见:

  Coverage.py
     A popular third-party coverage tool that provides HTML output
     along with advanced features such as branch coverage.


Command-Line Usage
==================

The "trace" module can be invoked from the command line.  It can be as
simple as

   python -m trace --count -C . somefile.py ...

The above will execute "somefile.py" and generate annotated listings
of all Python modules imported during the execution into the current
directory.

--help

   Display usage and exit.

--version

   Display the version of the module and exit.

3.8 新版功能: Added "--module" option that allows to run an executable
module.


Main options
------------

At least one of the following options must be specified when invoking
"trace".  The "--listfuncs" option is mutually exclusive with the "--
trace" and "--count" options. When "--listfuncs" is provided, neither
"--count" nor "--trace" are accepted, and vice versa.

-c, --count

   Produce a set of annotated listing files upon program completion
   that shows how many times each statement was executed.  See also "
   --coverdir", "--file" and "--no-report" below.

-t, --trace

   Display lines as they are executed.

-l, --listfuncs

   Display the functions executed by running the program.

-r, --report

   Produce an annotated list from an earlier program run that used the
   "--count" and "--file" option.  This does not execute any code.

-T, --trackcalls

   Display the calling relationships exposed by running the program.


Modifiers
---------

-f, --file=<file>

   Name of a file to accumulate counts over several tracing runs.
   Should be used with the "--count" option.

-C, --coverdir=<dir>

   Directory where the report files go.  The coverage report for
   "package.module" is written to file
   "*dir*/*package*/*module*.cover".

-m, --missing

   When generating annotated listings, mark lines which were not
   executed with ">>>>>>".

-s, --summary

   When using "--count" or "--report", write a brief summary to stdout
   for each file processed.

-R, --no-report

   Do not generate annotated listings.  This is useful if you intend
   to make several runs with "--count", and then produce a single set
   of annotated listings at the end.

-g, --timing

   Prefix each line with the time since the program started.  Only
   used while tracing.


Filters
-------

These options may be repeated multiple times.

--ignore-module=<mod>

   Ignore each of the given module names and its submodules (if it is
   a package).  The argument can be a list of names separated by a
   comma.

--ignore-dir=<dir>

   Ignore all modules and packages in the named directory and
   subdirectories. The argument can be a list of directories separated
   by "os.pathsep".


编程接口
========

class trace.Trace(count=1, trace=1, countfuncs=0, countcallers=0, ignoremods=(), ignoredirs=(), infile=None, outfile=None, timing=False)

   Create an object to trace execution of a single statement or
   expression.  All parameters are optional.  *count* enables counting
   of line numbers.  *trace* enables line execution tracing.
   *countfuncs* enables listing of the functions called during the
   run.  *countcallers* enables call relationship tracking.
   *ignoremods* is a list of modules or packages to ignore.
   *ignoredirs* is a list of directories whose modules or packages
   should be ignored.  *infile* is the name of the file from which to
   read stored count information.  *outfile* is the name of the file
   in which to write updated count information.  *timing* enables a
   timestamp relative to when tracing was started to be displayed.

      run(cmd)

         Execute the command and gather statistics from the execution
         with the current tracing parameters.  *cmd* must be a string
         or code object, suitable for passing into "exec()".

      runctx(cmd, globals=None, locals=None)

         Execute the command and gather statistics from the execution
         with the current tracing parameters, in the defined global
         and local environments.  If not defined, *globals* and
         *locals* default to empty dictionaries.

      runfunc(func, /, *args, **kwds)

         Call *func* with the given arguments under control of the
         "Trace" object with the current tracing parameters.

      results()

         Return a "CoverageResults" object that contains the
         cumulative results of all previous calls to "run", "runctx"
         and "runfunc" for the given "Trace" instance.  Does not reset
         the accumulated trace results.

class trace.CoverageResults

   A container for coverage results, created by "Trace.results()".
   Should not be created directly by the user.

      update(other)

         Merge in data from another "CoverageResults" object.

      write_results(show_missing=True, summary=False, coverdir=None)

         Write coverage results.  Set *show_missing* to show lines
         that had no hits.  Set *summary* to include in the output the
         coverage summary per module.  *coverdir* specifies the
         directory into which the coverage result files will be
         output.  If "None", the results for each source file are
         placed in its directory.

A simple example demonstrating the use of the programmatic interface:

   import sys
   import trace

   # create a Trace object, telling it what to ignore, and whether to
   # do tracing or line-counting or both.
   tracer = trace.Trace(
       ignoredirs=[sys.prefix, sys.exec_prefix],
       trace=0,
       count=1)

   # run the new command using the given tracer
   tracer.run('main()')

   # make a report, placing output in the current directory
   r = tracer.results()
   r.write_results(show_missing=True, coverdir=".")
