DbgE: Debugger with granularity of (sub-)expression

This repository provides an extension to IPdb to enable sub-expression stepping, bytecode stepping, advanced breakpoints and some stack manipulation.

DISCLAMER: This project is still in an experimental status and there is probably many gotchas that are not yet tackled.

Dependencies

• gcc and python headers for the C extension
• ipdb (pip install ipdb) for the base debugger

Extra commands

• stepe, steps until the next sub-expression
• stepi, steps until the next bytecode
• until_expr, let you interactively select an expression in the current debugged frame and run the execution until before the execution of this expression
• be expr, let you interactively select an expression in the function pointed by expr and sets a breakpoint there. The argument expr has to resolve to a function/method.
• breakattributeread [<mode>:]<expr>.<attr_name>, set a breakpoint on all attribute read access of a particular instance (see note for more details about modes).
• breakattributewrite [<mode>:]<expr>.<attr_name>, set a breakpoint on all attribute write access of a particular instance (see note for more details about modes).
• capturetopstack [arg], let you capture the top of the evaluation stack and place it in arg (_topstack if arg is not set) in the global context attached to the debugged frame.
• forcetopstack arg, forces the top of the evaluation stack to arg (experimental!)

Note about be

If the resolution is made against an instance, the breakpoint will be activated only before the execution of the interactively selected node for this exact instance. For example, let's consider the following code:

class A(object):
    def compute(self):
        print("Here")
        return 42

a1 = A()
a2 = A()

import dbge; dbge.set_trace()

a1.compute()
a2.compute()

In the debugger, calling be A.compute will let you set a breakpoint on an expression inside of compute. Consequently, the execution will break before the execution of the selected sub-expression when a1.compute() and a2.compute() will be executed. However, calling be a2.compute will let you set a breakpoint on an expression inside of compute that will trigger only when compute will be executed for a2. This enables a first object-centric breakpoint type.

Note about breakattribute[read/write]

Those two commands let you set a breakpoint for a dedicated instance (object-centric breakpoint) that will activate on an attribute access (read or write). The command argument format is [<mode>:]<expr>.<attr_name>, and each part where:

• <expr> is an expression that needs to resolve to an object (e.g: in example.py, it could be a1 or any other complex expression that resolve to an object accessible from the frame being debugged).
• <attr_name> is the name of the attribute to break to, e.g: a1.name would consider the attribute name of the instance a1.
• <mode> is in which "situation" the breakpoint should activate. It can be:
  • internal (or i): it will activate only on an attribute read/write that is performed in a method of the instance itself
  • external (or e): it will activate only on an attribute read/write that is performed "outside" of the instance (another object in another methode/function access or modifies the attribute)
  • both (or b): the default value if mode is not set. It will activate on an attribute read/write performed "inside" or "outside" of the instance.

How to test the "example.py"

The project is not yet on pypi (not sure it will be one day) as it's experimental and contains a C extension.

Is there the way of playing with the example.py

1. clone the repository somewhere and go inside

git clone https://github.com/aranega/dbge
cd dbge

2. compile the C extension to gain access to some evaluation stack manipulation

make   # requires the cpython dev package installed, as well as gcc

3. run the example.py or set import dbge; dgbe.set_trace() where you want in your code.

Demo videos

General use of stepi, stepe, until_expr, forcetopstack and capturetopstack	Use of expression breakpoint with be
presentation2.mp4	presentation_be.mp4