Lib/pdb.py (part 2)

Source:

cpython 3.14 @ ab2d84fe1023/Lib/pdb.py ↗

This annotation covers pdb command implementations. See lib_pdb_detail for Pdb.__init__, set_trace, the cmd.Cmd base class, and breakpoint management.

Map

Lines	Symbol	Role
1-100	Pdb.do_next (n)	Step over; continue until the next line in current frame
101-220	Pdb.do_step (s)	Step into; stop at the next line in any frame
221-360	Pdb.do_break (b)	Set a breakpoint at a line or function
361-480	Pdb.do_continue (c)	Resume execution; clear step flags
481-600	Pdb.do_print (p)	Evaluate and print an expression in the current frame

Reading

Pdb.do_next

# CPython: Lib/pdb.py:1580 Pdb.do_next
def do_next(self, arg):
    """n(ext): Continue execution until the next line in the current frame."""
    if arg:
        self._next_count = int(arg)
    self.set_next(self.curframe)
    return 1  # Signal cmdloop to return (resume execution)

def set_next(self, frame):
    """Stop on the next line in the given frame or any frame that returns to it."""
    self.stopframe = frame
    self.returnframe = frame
    self.set_step()
    frame.f_trace_lines = True

set_next sets stopframe so that trace_dispatch stops only when execution returns to the current frame or advances a line within it. The trace function is set to line mode only for the target frame.

Pdb.do_break

# CPython: Lib/pdb.py:1040 Pdb.do_break
def do_break(self, arg, temporary=False):
    """b(reak) [loc [, condition]]: Set a breakpoint."""
    ...
    # Parse filename:lineno or function name
    bp = self.set_break(filename, lineno, temporary, cond, funcname)
    if isinstance(bp, str):  # Error message
        self.error(bp)
    else:
        self.message(f"Breakpoint {bp.number} at {filename}:{lineno}")

Breakpoints are stored in bdb.Breakpoint.bplist (a dict keyed by (filename, lineno)). The trace function checks bdb.effective on each line event to see if any breakpoint is hit.

Pdb.do_print

# CPython: Lib/pdb.py:1780 Pdb.do_print
def do_print(self, arg):
    """p expression: Print the value of the expression."""
    if not arg:
        self.error('Missing argument to print command')
        return
    try:
        val = self._getval(arg)
    except:
        return  # Error already printed
    self.message(repr(val))

def _getval(self, arg):
    try:
        return eval(arg, self.curframe.f_globals, self.curframe_locals)
    except:
        exc = sys.exception()
        self.error(traceback.format_exception_only(exc)[-1].strip())
        raise

p expr evaluates expr in curframe.f_globals and curframe_locals (the latter calls _PyFrame_FastToLocals). Exceptions during eval are caught and displayed without crashing pdb.

gopy notes

pdb is not yet fully ported in gopy. set_trace() is module/pdb.SetTrace in module/pdb/module.go, which installs vm.TraceFunction. do_next/do_step set vm.Frame.StopFrame. do_break stores breakpoints in module/bdb.BreakpointList. do_print calls vm.EvalExpr in the current frame's namespace.