# 27.6. `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.

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Coverage.py

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

## 27.6.1. 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.

### 27.6.1.1. 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.

### 27.6.1.2. 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.

### 27.6.1.3. 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`.

## 27.6.2. Programmatic Interface

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=".")