bdb — Debugger framework

Source code: Lib/bdb.py

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=False, 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 file name 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:

  • Breakpoint number.

  • Temporary status (del or keep).

  • File/line position.

  • Break condition.

  • Number of times to ignore.

  • Number of times hit.

Added in version 3.2.

bpprint(out=None)

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

Breakpoint instances have the following attributes:

file

File name of the Breakpoint.

line

Line number of the Breakpoint within file.

temporary

True if a Breakpoint at (file, line) is temporary.

cond

Condition for evaluating a Breakpoint at (file, line).

funcname

Function name that defines whether a Breakpoint is hit upon entering the function.

enabled

True if Breakpoint is enabled.

bpbynumber

Numeric index for a single instance of a Breakpoint.

bplist

Dictionary of Breakpoint instances indexed by (file, line) tuples.

ignore

Number of times to ignore a Breakpoint.

hits

Count of the number of times a Breakpoint has been hit.

class bdb.Bdb(skip=None, backend='settrace')

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.

The backend argument specifies the backend to use for Bdb. It can be either 'settrace' or 'monitoring'. 'settrace' uses sys.settrace() which has the best backward compatibility. The 'monitoring' backend uses the new sys.monitoring that was introduced in Python 3.12, which can be much more efficient because it can disable unused events. We are trying to keep the exact interfaces for both backends, but there are some differences. The debugger developers are encouraged to use the 'monitoring' backend to achieve better performance.

Changed in version 3.1: Added the skip parameter.

Changed in version 3.14: Added the backend parameter.

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

canonic(filename)

Return canonical form of filename.

For real file names, the canonical form is an operating-system-dependent, case-normalized absolute path. A filename with angle brackets, such as "<stdin>" generated in interactive mode, is returned unchanged.

start_trace(self)

Start tracing. For 'settrace' backend, this method is equivalent to sys.settrace(self.trace_dispatch)

Added in version 3.14.

stop_trace(self)

Stop tracing. For 'settrace' backend, this method is equivalent to sys.settrace(None)

Added in version 3.14.

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.

For all the events, specialized functions (see below) are called.

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 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 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 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 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.

is_skipped_line(module_name)

Return True if module_name matches any skip pattern.

stop_here(frame)

Return True if frame is below the starting frame in the stack.

break_here(frame)

Return True if there is an effective breakpoint for this line.

Check whether a line or function breakpoint exists and is in effect. Delete temporary breakpoints based on information from effective().

break_anywhere(frame)

Return True if any breakpoint exists for frame’s filename.

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

user_call(frame, argument_list)

Called from dispatch_call() if a break might stop inside the called function.

argument_list is not used anymore and will always be None. The argument is kept for backwards compatibility.

user_line(frame)

Called from dispatch_line() when either stop_here() or break_here() returns True.

user_return(frame, return_value)

Called from dispatch_return() when stop_here() returns True.

user_exception(frame, exc_info)

Called from dispatch_exception() when stop_here() returns 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, lineno=None)

Stop when the line with the lineno 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.

Changed in version 3.13: set_trace() will enter the debugger immediately, rather than on the next line of code to be executed.

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=False, cond=None, funcname=None)

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, return an error message.

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, return an error message.

clear_all_breaks()

Delete all existing breakpoints. If none were set, return an error message.

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.

Added in version 3.2.

get_break(filename, lineno)

Return True if there is a breakpoint for lineno in 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 disable and restart events to achieve better performance. These methods only work when using the 'monitoring' backend.

disable_current_event()

Disable the current event until the next time restart_events() is called. This is helpful when the debugger is not interested in the current line.

Added in version 3.14.

restart_events()

Restart all the disabled events. This function is automatically called in dispatch_* methods after user_* methods are called. If the dispatch_* methods are not overridden, the disabled events will be restarted after each user interaction.

Added in version 3.14.

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

get_stack(f, t)

Return a list of (frame, lineno) tuples in a stack trace, and a size.

The most recently called frame is last in the list. The size is the number of frames below the frame where the debugger was invoked.

format_stack_entry(frame_lineno, lprefix=': ')

Return a string with information about a stack entry, which is a (frame, lineno) tuple. The return string contains:

  • The canonical 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)

Return True if 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 frame. 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 on its first executable line.

bdb.effective(file, line, frame)

Return (active breakpoint, delete temporary flag) or (None, None) as the breakpoint to act upon.

The active breakpoint is the first entry in bplist for the (file, line) (which must exist) that is enabled, for which checkfuncname() is true, and that has neither a false condition nor positive ignore count. The flag, meaning that a temporary breakpoint should be deleted, is False only when the cond cannot be evaluated (in which case, ignore count is ignored).

If no such entry exists, then (None, None) is returned.

bdb.set_trace()

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