27.1. "bdb" — Debugger framework
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**Source code:** Lib/bdb.py

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The "bdb" module handles basic debugger functions, like setting
breakpoints or managing execution via the debugger.

The following exception is defined:

exception bdb.BdbQuit

   Exception raised by the "Bdb" class for quitting the debugger.

The "bdb" module also defines two classes:

class bdb.Breakpoint(self, file, line, temporary=0, cond=None, funcname=None)

   This class implements temporary breakpoints, ignore counts,
   disabling and (re-)enabling, and conditionals.

   Breakpoints are indexed by number through a list called
   "bpbynumber" and by "(file, line)" pairs through "bplist".  The
   former points to a single instance of class "Breakpoint".  The
   latter points to a list of such instances since there may be more
   than one breakpoint per line.

   When creating a breakpoint, its associated filename should be in
   canonical form.  If a *funcname* is defined, a breakpoint hit will
   be counted when the first line of that function is executed.  A
   conditional breakpoint always counts a hit.

   "Breakpoint" instances have the following methods:

   deleteMe()

      Delete the breakpoint from the list associated to a file/line.
      If it is the last breakpoint in that position, it also deletes
      the entry for the file/line.

   enable()

      Mark the breakpoint as enabled.

   disable()

      Mark the breakpoint as disabled.

   bpformat()

      Return a string with all the information about the breakpoint,
      nicely formatted:

      * The breakpoint number.

      * If it is temporary or not.

      * Its file,line position.

      * The condition that causes a break.

      * If it must be ignored the next N times.

      * The breakpoint hit count.

      New in version 3.2.

   bpprint(out=None)

      Print the output of "bpformat()" to the file *out*, or if it is
      "None", to standard output.

class bdb.Bdb(skip=None)

   The "Bdb" class acts as a generic Python debugger base class.

   This class takes care of the details of the trace facility; a
   derived class should implement user interaction.  The standard
   debugger class ("pdb.Pdb") is an example.

   The *skip* argument, if given, must be an iterable of glob-style
   module name patterns.  The debugger will not step into frames that
   originate in a module that matches one of these patterns. Whether a
   frame is considered to originate in a certain module is determined
   by the "__name__" in the frame globals.

   New in version 3.1: The *skip* argument.

   The following methods of "Bdb" normally don’t need to be
   overridden.

   canonic(filename)

      Auxiliary method for getting a filename in a canonical form,
      that is, as a case-normalized (on case-insensitive filesystems)
      absolute path, stripped of surrounding angle brackets.

   reset()

      Set the "botframe", "stopframe", "returnframe" and "quitting"
      attributes with values ready to start debugging.

   trace_dispatch(frame, event, arg)

      This function is installed as the trace function of debugged
      frames.  Its return value is the new trace function (in most
      cases, that is, itself).

      The default implementation decides how to dispatch a frame,
      depending on the type of event (passed as a string) that is
      about to be executed. *event* can be one of the following:

      * ""line"": A new line of code is going to be executed.

      * ""call"": A function is about to be called, or another code
        block entered.

      * ""return"": A function or other code block is about to return.

      * ""exception"": An exception has occurred.

      * ""c_call"": A C function is about to be called.

      * ""c_return"": A C function has returned.

      * ""c_exception"": A C function has raised an exception.

      For the Python events, specialized functions (see below) are
      called.  For the C events, no action is taken.

      The *arg* parameter depends on the previous event.

      See the documentation for "sys.settrace()" for more information
      on the trace function.  For more information on code and frame
      objects, refer to The standard type hierarchy.

   dispatch_line(frame)

      If the debugger should stop on the current line, invoke the
      "user_line()" method (which should be overridden in subclasses).
      Raise a "BdbQuit" exception if the "Bdb.quitting" flag is set
      (which can be set from "user_line()").  Return a reference to
      the "trace_dispatch()" method for further tracing in that scope.

   dispatch_call(frame, arg)

      If the debugger should stop on this function call, invoke the
      "user_call()" method (which should be overridden in subclasses).
      Raise a "BdbQuit" exception if the "Bdb.quitting" flag is set
      (which can be set from "user_call()").  Return a reference to
      the "trace_dispatch()" method for further tracing in that scope.

   dispatch_return(frame, arg)

      If the debugger should stop on this function return, invoke the
      "user_return()" method (which should be overridden in
      subclasses). Raise a "BdbQuit" exception if the "Bdb.quitting"
      flag is set (which can be set from "user_return()").  Return a
      reference to the "trace_dispatch()" method for further tracing
      in that scope.

   dispatch_exception(frame, arg)

      If the debugger should stop at this exception, invokes the
      "user_exception()" method (which should be overridden in
      subclasses). Raise a "BdbQuit" exception if the "Bdb.quitting"
      flag is set (which can be set from "user_exception()").  Return
      a reference to the "trace_dispatch()" method for further tracing
      in that scope.

   Normally derived classes don’t override the following methods, but
   they may if they want to redefine the definition of stopping and
   breakpoints.

   stop_here(frame)

      This method checks if the *frame* is somewhere below "botframe"
      in the call stack.  "botframe" is the frame in which debugging
      started.

   break_here(frame)

      This method checks if there is a breakpoint in the filename and
      line belonging to *frame* or, at least, in the current function.
      If the breakpoint is a temporary one, this method deletes it.

   break_anywhere(frame)

      This method checks if there is a breakpoint in the filename of
      the current frame.

   Derived classes should override these methods to gain control over
   debugger operation.

   user_call(frame, argument_list)

      This method is called from "dispatch_call()" when there is the
      possibility that a break might be necessary anywhere inside the
      called function.

   user_line(frame)

      This method is called from "dispatch_line()" when either
      "stop_here()" or "break_here()" yields "True".

   user_return(frame, return_value)

      This method is called from "dispatch_return()" when
      "stop_here()" yields "True".

   user_exception(frame, exc_info)

      This method is called from "dispatch_exception()" when
      "stop_here()" yields "True".

   do_clear(arg)

      Handle how a breakpoint must be removed when it is a temporary
      one.

      This method must be implemented by derived classes.

   Derived classes and clients can call the following methods to
   affect the stepping state.

   set_step()

      Stop after one line of code.

   set_next(frame)

      Stop on the next line in or below the given frame.

   set_return(frame)

      Stop when returning from the given frame.

   set_until(frame)

      Stop when the line with the line no greater than the current one
      is reached or when returning from current frame.

   set_trace([frame])

      Start debugging from *frame*.  If *frame* is not specified,
      debugging starts from caller’s frame.

   set_continue()

      Stop only at breakpoints or when finished.  If there are no
      breakpoints, set the system trace function to "None".

   set_quit()

      Set the "quitting" attribute to "True".  This raises "BdbQuit"
      in the next call to one of the "dispatch_*()" methods.

   Derived classes and clients can call the following methods to
   manipulate breakpoints.  These methods return a string containing
   an error message if something went wrong, or "None" if all is well.

   set_break(filename, lineno, temporary=0, cond, funcname)

      Set a new breakpoint.  If the *lineno* line doesn’t exist for
      the *filename* passed as argument, return an error message.  The
      *filename* should be in canonical form, as described in the
      "canonic()" method.

   clear_break(filename, lineno)

      Delete the breakpoints in *filename* and *lineno*.  If none were
      set, an error message is returned.

   clear_bpbynumber(arg)

      Delete the breakpoint which has the index *arg* in the
      "Breakpoint.bpbynumber".  If *arg* is not numeric or out of
      range, return an error message.

   clear_all_file_breaks(filename)

      Delete all breakpoints in *filename*.  If none were set, an
      error message is returned.

   clear_all_breaks()

      Delete all existing breakpoints.

   get_bpbynumber(arg)

      Return a breakpoint specified by the given number.  If *arg* is
      a string, it will be converted to a number.  If *arg* is a non-
      numeric string, if the given breakpoint never existed or has
      been deleted, a "ValueError" is raised.

      New in version 3.2.

   get_break(filename, lineno)

      Check if there is a breakpoint for *lineno* of *filename*.

   get_breaks(filename, lineno)

      Return all breakpoints for *lineno* in *filename*, or an empty
      list if none are set.

   get_file_breaks(filename)

      Return all breakpoints in *filename*, or an empty list if none
      are set.

   get_all_breaks()

      Return all breakpoints that are set.

   Derived classes and clients can call the following methods to get a
   data structure representing a stack trace.

   get_stack(f, t)

      Get a list of records for a frame and all higher (calling) and
      lower frames, and the size of the higher part.

   format_stack_entry(frame_lineno, lprefix=': ')

      Return a string with information about a stack entry, identified
      by a "(frame, lineno)" tuple:

      * The canonical form of the filename which contains the frame.

      * The function name, or ""<lambda>"".

      * The input arguments.

      * The return value.

      * The line of code (if it exists).

   The following two methods can be called by clients to use a
   debugger to debug a *statement*, given as a string.

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

      Debug a statement executed via the "exec()" function.  *globals*
      defaults to "__main__.__dict__", *locals* defaults to *globals*.

   runeval(expr, globals=None, locals=None)

      Debug an expression executed via the "eval()" function.
      *globals* and *locals* have the same meaning as in "run()".

   runctx(cmd, globals, locals)

      For backwards compatibility.  Calls the "run()" method.

   runcall(func, *args, **kwds)

      Debug a single function call, and return its result.

Finally, the module defines the following functions:

bdb.checkfuncname(b, frame)

   Check whether we should break here, depending on the way the
   breakpoint *b* was set.

   If it was set via line number, it checks if "b.line" is the same as
   the one in the frame also passed as argument.  If the breakpoint
   was set via function name, we have to check we are in the right
   frame (the right function) and if we are in its first executable
   line.

bdb.effective(file, line, frame)

   Determine if there is an effective (active) breakpoint at this line
   of code. Return a tuple of the breakpoint and a boolean that
   indicates if it is ok to delete a temporary breakpoint.  Return
   "(None, None)" if there is no matching breakpoint.

bdb.set_trace()

   Start debugging with a "Bdb" instance from caller’s frame.
