pyfields.py

debugging python object fields with gdb

$ python foo.py &
[1] 21377
$ gdb -p 21377
GNU gdb (GDB) Fedora 7.7.1-17.fc20
[... gdb loading messages ...]
0x00007f9a09af46e3 in __select_nocancel () at ../sysdeps/unix/syscall-template.S:81
81	T_PSEUDO (SYSCALL_SYMBOL, SYSCALL_NAME, SYSCALL_NARGS)
(gdb) py-bt
#4 Frame 0x22d9c70, for file foo.py, line 12, in forever ()
    sleep(1)
#7 Frame 0x224f830, for file foo.py, line 14, in <module> ()
    forever()
(gdb) py-print f
global 'f' = <Foo(foo=1) at remote 0x7f9a0abba830>
(gdb) py-print f.foo
'f.foo' not found
(gdb) source pyfields.py 
(gdb) py-fields f.foo
global 'foo' = 1
(gdb) quit
A debugging session is active.

	Inferior 1 [process 21377] will be detached.

Quit anyway? (y or n) y
Detaching from program: /usr/bin/python2.7, process 21377
$ exit

foo.py

from time import sleep


class Foo:
   def __init__(self):
      self.foo = 1

f = Foo()

def forever():
    while f.foo == 1:
        sleep(1)

forever()

pyfields.py

class PyPrintFields(gdb.Command):
    'Look up the given python variable name, and print it'
    def __init__(self):
        gdb.Command.__init__ (self,
                              "py-fields",
                              gdb.COMMAND_DATA,
                              gdb.COMPLETE_NONE)


    def invoke(self, args, from_tty):
        name = str(args)

        frame = Frame.get_selected_python_frame()
        if not frame:
            print 'Unable to locate python frame'
            return

        pyop_frame = frame.get_pyop()
        if not pyop_frame:
            print 'Unable to read information on python frame'
            return

        def get_var_dict(v):
            if isinstance(v, PyInstanceObjectPtr):
                return v.pyop_field('in_dict')
            elif isinstance(v, HeapTypeObjectPtr):
                return v.get_attr_dict()
            else:
                return {}
        parent = None
        fields = name.split('.')
        for name in fields:
            if parent is None:
                pyop_var, scope = pyop_frame.get_var_by_name(name)
            else:
                for k, pyop_var in get_var_dict(parent).iteritems():
                    if str(k) == name:
                        break
                else:
                    print "Can't find field %s of %s" % (name, parent)
                    return
            parent = pyop_var
        if pyop_var:
            print ('%s %r = %s'
                   % (scope,
                      name,
                      pyop_var.get_truncated_repr(MAX_OUTPUT_LEN)))
        else:
            print '%r not found' % name

PyPrintFields()