NoPdb: Non-interactive Python Debugger


NoPdb: Non-interactive Python Debugger

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NoPdb is a programmatic (non-interactive) debugger for Python. This means it gives you access to debugger-like superpowers directly from your code. With NoPdb, you can:

  • capture function calls, including arguments, local variables, return values and stack traces
  • set "breakpoints" that trigger user-defined actions when hit, namely:
    • evaluate expressions to retrieve their values later
    • execute arbitrary code, including modifying local variables
    • enter an interactive debugger like pdb

NoPdb is also a convenient tool for inspecting machine learning model internals. For example, this notebook shows how to use it to visualize Transformer attention in PyTorch.

NoPdb should run at least under CPython and PyPy. Most features should work under any implementation as long as it has sys.settrace().

Note: This project is in its early development stage. Contributions and improvement ideas are welcome.

Capturing function calls

The functions capture_call() and capture_calls() allow capturing useful information about calls to a given function. They are typically used as context managers, e.g.:

with nopdb.capture_calls(fn) as calls:

    print(calls)  # see details about how fn() was called

The information we can retrieve includes the function's arguments, return value, local variables and stack trace. For example:

>>> with nopdb.capture_call(f) as call:
...     g(1)
>>> call
CallCapture(name='f', args=OrderedDict(x=1, y=1), return_value=4)
>>> call.print_stack()
  File "<stdin>", line 2, in <module>
  File "<stdin>", line 2, in g
  File "<stdin>", line 1, in f
>>> call.args['x']
>>> call.return_value
>>> call.locals
{'y': 1, 'x': 1, 'z': 2}

Setting breakpoints

Like conventional debuggers, NoPdb can set breakpoints. However, because NoPdb is a non-interactive debugger, its breakpoints do not actually stop the execution of the program. Instead, they allow executing actions scheduled in advance, such as evaluating expressions.

To set a breakpoint, call the breakpoint() function. A breakpoint object is returned, allowing to schedule actions using its methods such as eval() and exec(). For example:

# Break at line 3 of the file or notebook cell where f is defined
with nopdb.breakpoint(function=f, line=3) as bp:
    x = bp.eval("x")             # Schedule an expression
    type_y = bp.eval("type(y)")  # Another one
    bp.exec("print(y)")          # Schedule a print statement


print(x, type_y)  # Retrieve the captured values

There are other ways to specify the breakpoint location. For example:

# Break at any line with the given source code in the given file
with nopdb.breakpoint(file="", line="return obj") as bp:

# Break as soon as any function with the given name is called
with nopdb.breakpoint(function="myfunc") as bp:

Not only can we capture values, we can also modify them!

>>> with nopdb.breakpoint(function=f, line=3) as bp:
...     # Get the value of x, then increment it, then get the new value
...     x_before = bp.eval('x')
...     bp.exec('x += 1')
...     x_after = bp.eval('x')
...     some_code_that_calls_f()
>>> x_before
>>> x_after

Planned features

Functionalities that do not exist, but could be added in the future:

  • Breakpoint.callback() for calling a given callback function, passing information about the current frame as an argument.
  • Breakpoint.jump() for jumping to a different line in the same function.
  • A way to disable breakpoints.


  • Like Pdb, NoPdb only works with pure-Python functions. Calls to built-ins and C extensions cannot be captured. This also applies to ML frameworks that compile models into static graphs; for NoPdb to work, this feature needs to be disabled, e.g. with tf.config.run_functions_eagerly(True) in TensorFlow and with the jax.disable_jit() context manager in JAX.
  • Local variable assignment in Breakpoint.exec() is only supported under CPython and PyPy.
  • Functionality questions

    Functionality questions

    Thanks for making this project. A non-interactive python debugger is exactly what I'm looking for. I'd like to use your project to support the Python probe for Source++. I've been able to successfully implement some portions of the functionality, but I'm having some trouble understanding some Python debugging relating things. I think some of this trouble might be alleviated with new functionality for nopdb and I'm more than willing to implement that functionality. My background is with JVM languages so I'm just looking for a bit of guidance on how things work the Python way.

    Breakpoint doesn't hit on Flask app

    I don't think this issue is a nopdb issue as I'm unable to hit the breakpoint using import pdb; pdb.set_trace() in the main method either. I believe this issue is because of either compiled code within Flask or pdb is not hitting because of separate threads. Either way, I'm able to set breakpoints via PyCharm so it appears to be possible in some way. Hopefully, it's just a threading thing and breakpoints can be placed for every thread. Do you know how this functionality could be implemented with nopdb?

    Here is some code that shows a breakpoint that's never hit:

    import os
    from flask import Flask
    from nopdb import nopdb
    app = Flask(__name__)
    def hello():
        provider = str(os.environ.get('PROVIDER', 'world'))
        return 'Hello ' + provider + '!'
    if __name__ == '__main__':
        with nopdb.breakpoint(file="", line=12) as bp:
            bp.exec("print(\"provider: \" + provider)")
            port = int(os.environ.get('PORT', 5000))
  '', port=port)

    Ability to add breakpoints without calling code via context manager

    This might be related to the issue above, but how do you add breakpoints to code that you don't want to call via the context manager? As in, code that is already running or that will run by itself sometime in the future? I have a small example here where I want to set a breakpoint before a function is called in the future via a delay. Can nopdb address this type of scenario?

    Code which shows a breakpoint that's never hit:

    import threading
    from nopdb import nopdb
    def fun():
        x = 5
        y = x
    if __name__ == '__main__':
        start_time = threading.Timer(10, fun)
       # I'd like to set the breakpoint ahead of time and just output x once it's hit
        with nopdb.breakpoint(file="", line=8) as bp:
            bp.exec("print(\"x: \" + str(x))")

    Does cpdb make sense to extend?

    This might be a bit off-topic, but while looking into Python debuggers I ran across Does this project make sense to extend for nopdb? The reason I ask is because of the claim cpdb makes about "long-running" applications running better with it. I would like to use nopdb in "long-running" applications so I'm curious if it makes sense to integrate the technologies.

    opened by BFergerson 0
Ondřej Cífka
PhD candidate @ Télécom Paris. Working on music style transfer
Ondřej Cífka
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