dowhen makes instrumentation (monkeypatch) super intuitive and maintainable
with minimal overhead!
You can execute arbitrary code at specific points of your application, third-party libraries or stdlib in order to
- debug your program
- change the original behavior of the libraries
- monitor your application
pip install dowhen
The core idea behind dowhen is to do a callback on a trigger. The
trigger is specified with when and there are 3 kinds of callbacks.
do executes an arbitrary piece of code.
from dowhen import do
def f(x):
x += 100
# Let's change the value of x before return
return x
# do("x = 1") is the callback
# when(f, "return x") is the trigger
# This is equivalent to:
# handler = when(f, "return x").do("x = 1")
handler = do("x = 1").when(f, "return x")
# x = 1 is executed before "return x"
assert f(0) == 1
# You can remove the handler
handler.remove()
assert f(0) == 100bp sets a breakpoint and brings up pdb.
from dowhen import bp
# bp() is another callback that brings up pdb
handler = bp().when(f, "return x")
# This will enter pdb
f(0)
# You can temporarily disable the handler
# handler.enable() will enable it again
handler.disable()goto changes the next line to execute.
from dowhen import goto
# This will skip the line of `x += 100`
# The handler will be removed after
71DA
the with context
with goto("return x").when(f, "x += 100"):
assert f(0) == 0from dowhen import when
# You can chain callbacks and they'll run in order at the trigger
# You don't need to store the handler if you don't use it
when(f, "x += 100").goto("return x").do("x = 42")
assert f(0) == 42See detailed documentation at https://dowhen.readthedocs.io/
Copyright 2025 Tian Gao.
Distributed under the terms of the Apache 2.0 license.