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Python module to facilitate and automate the process of writing runtime logs.

Project description

PyLg (read as py-log) is a python module that facilitates the process of writing runtime logs. The goal of PyLg is to provide an unobtrusive and flexible interface that automates the process of generating informative logs.



  • Ease of use - the API consists of only one decorator and one function.
  • Flexible - the user can set global preferences as well as on a per-function basis.
  • Informative - PyLg can automatically log input arguments, return values and exceptions raised.
  • User logs - the user can make additional logs that will be collected together with the automatically generated logs.


[sudo] pip install pylg --upgrade

Note that PyLg is under active development. Frequent upgrades are recommended.


Import the module:

from pylg import TraceFunction, trace

To automatically log function entry and exit use the @TraceFunction decorator:

def some_fuction():

Despite the name, this works for both functions and methods.

@TraceFunction can take up to seven optional arguments:

  • exception_warning - if True, PyLg will print a warning about every exception caught to stderr.
  • exception_tb_file - if True, PyLg will write the exception tracebacks to the log file.
  • exception_tb_stderr - if True, PyLg will print the exception tracebacks to stderr.
  • exception_exit - if True, PyLg will force the program to exit (and not just raise SystemExit) whenever an exception occurs. This will happen even if the exception would be handled at a later point.
  • trace_args - if True, PyLg will log input parameters.
  • trace_rv - if True, PyLg will log return values.
  • trace_rv_type - if True, PyLg will log return value types.

The default values for these arguments are set in a global settings file.

These arguments have to specified explicitly by name. Some examples:

@TraceFunction(trace_args = False)
def some_fuction():

@TraceFunction(trace_args = False, exception_tb_stderr = True)
def some_fuction():

The other way to interact with PyLg is to log a user defined message with the trace function.

trace("The user can pass any string they desire in here")

User Settings

The user can adjust several settings to suit their preferences. To do so, create a file named somewhere in your path and set any of the following variables to the desired values in order to override the defaults. The file in the project directory contains all the default settings and can be used as a template.

  • PYLG_ENABLE (default = True) - enable/disable PyLg.
  • PYLG_FILE (default = 'pylg.log') - the log file name.
  • DEFAULT_EXCEPTION_WARNING (default = True) - the default setting for exception_warning.
  • DEFAULT_EXCEPTION_TB_FILE (default = True) - the default setting for exception_tb_file.
  • DEFAULT_EXCEPTION_TB_STDERR (default = False) - the default setting for exception_tb_stderr.
  • DEFAULT_EXCEPTION_EXIT (default = False) - the default setting for exception_exit.
  • TRACE_TIME (default = TRUE) - enable/disable time logging.
  • TIME_FORMAT (default = "%Y-%m-%d %H:%M:%S.%f") - formatting for the time trace. For a full list of options, see
  • TRACE_FILENAME (default = True) - enable/disable file name logging.
  • FILENAME_COLUMN_WIDTH (default = 20) - the column width for the file name. If a name is too long, it will be truncated.
  • TRACE_LINENO (default = True) - enable/disable the logging of the line number from which the trace call was made. For entry and exit messages this logs the line in which the decorator is placed (which should be directly above the function itself).
  • LINENO_WIDTH (default = 4) - the minimum number of digits to use to print the line number. If the number is too long, more digits will be used.
  • TRACE_FUNCTION (default = True) - enable/disable the logging of the function name from which the trace call was made. Entry/exit logs refer to the function they enter into and exit from.
  • FUNCTION_COLUMN_WIDTH (default = 32) - the column width for the function name. If a name is too long, it will be truncated.
  • CLASS_NAME_RESOLUTION (default = False) - enable/disable class name resolution. Function names will be printed with their class names. IMPORTANT: If this setting is enabled, the trace function should ONLY be called from within functions that have the @TraceFunction decorator OR outside of any function.
  • TRACE_MESSAGE (default = True) - enable/disable message logging.
  • MESSAGE_WIDTH (default = 0) - the column width for the message. A width of zero means unlimited.
  • MESSAGE_WRAP (default = True) - if True, PyLg will wrap the message to fit within the column width. Otherwise, the message will be truncated.
  • MESSAGE_MARK_TRUNCATION (default = True) - if True, truncated message lines should have the last character replaced with \. Note that this reduces MESSAGE_WIDTH by 1 for truncated lines which may truncate words that would’ve otherwise appeared in the message.
  • TRACE_SELF (default = False) - enable/disable logging of the self function argument.
  • COLLAPSE_LISTS (default = False) - if True lists will be collapsed to [ len=x ] where x denotes the number of elements in the list.
  • COLLAPSE_DICTS (default = False) - if True dictionaries will be collapsed to { len=x } where x denotes the number of elements in the dictionary.
  • DEFAULT_TRACE_ARGS (default = True) - the default setting for trace_args.
  • DEFAULT_TRACE_RV (default = True) - the default setting for trace_rv.
  • DEFAULT_TRACE_RV_TYPE (default = True) - the default setting for trace_rv_type.

Under development

Since this module is under development, here are a few things to keep in mind when using PyLg.

  • The behaviour of @TraceFunction has not been tested when multiple decorators are present.
  • When PyLg opens a new log file, it overwrites any file present with the same name. Therefore, it can erase important files if you are not careful.
  • Some features of PyLg do not work with old-style classes.


Please submit contributions branched from the pylg-dev branch.

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