watchpoints is an easy-to-use, intuitive variable/object monitor tool for python that behaves similar to watchpoints in gdb.

pip install watchpoints

Simply watch the variables you need to monitor!

from watchpoints import watch

a = 0
watch(a)
a = 1

will generate

====== Watchpoints Triggered ======
Call Stack (most recent call last):
  <module> (my_script.py:5):
>   a = 1
a:
0
->
1

It works on both variable change and object change

from watchpoints import watch

a = []
watch(a)
a.append(1)  # Trigger
a = {}  # Trigger

Even better, it can track the changes of the object after the changes of the variable

from watchpoints import watch

a = []
watch(a)
a = {}  # Trigger
a["a"] = 2  # Trigger

Without doubts, it works whenever the object is changed, even if it's not in the same scope

from watchpoints import watch

def func(var):
    var["a"] = 1

a = {}
watch(a)
func(a)

====== Watchpoints Triggered ======
Call Stack (most recent call last):
  <module> (my_script.py:8):
>   func(a)
  func (my_script.py:4):
>   var["a"] = 1
a:
{}
->
{'a': 1}

As you can imagine, you can monitor attributes of an object, or a specific element of a list or a dict

from watchpoints import watch

class MyObj:
    def __init__(self):
        self.a = 0

obj = MyObj()
d = {"a": 0}
watch(obj.a, d["a"])  # Yes you can do this
obj.a = 1  # Trigger
d["a"] = 1  # Trigger

Also, watchpoints supports native threading library for multi-threading. It will tell you which thread is changing the value as well.

====== Watchpoints Triggered ======
---- Thread-1 ----
Call Stack (most recent call last):
  _bootstrap (/usr/lib/python3.8/threading.py:890):
>   self._bootstrap_inner()
  _bootstrap_inner (/usr/lib/python3.8/threading.py:932):
>   self.run()
  run (my_script.py:15):
>   a[0] = i
a:
[0]
->
[1]

watchpoints will try to guess what you want to monitor, and monitor it as you expect(well most of the time)

When you are done with the variable, you can unwatch it.

from watchpoints import watch, unwatch

a = 0
watch(a)
a = 1
unwatch(a)
a = 2  # nothing will happen

Or you can unwatch everything by passing no argument to it

unwatch()  # unwatch everything

Like the print function, you can choose the output stream for watch print using file argument. The default value is sys.stderr.

f = open("watch.log", "w")
a = 0
watch(a, file=f)
a = 1
f.close()

Be aware that the stream needs to be available when the variable is changed! So the following code WON'T WORK:

a = 0
with open("watch.log", "w") as f:
    watch(a, file=f)
a = 1

Or you could just give a filename to watch. It will append to the file.

watch(a, file="watch.log")

Use config if you want to make it global

watch.config(file="watch.log")

You can use your own printer function to print the object, instead of the default objprint with custom_printer

# This will use built-in print function for the objects
watch(a, custom_printer=print)

Use config if you want to make it global

watch.config(custom_printer=print)

You can give an alias to a monitored variable, so you can unwatch it anywhere. And the alias will be printed instead of the variable name

from watchpoints import watch, unwatch

watch(a, alias="james")
# Many other stuff, scope changes
unwatch("james")

You can give an extra condition filter to do "conditional watchpoints". Pass a function func(obj) which returns True if you want to trigger the callback to when of watch

a = 0
watch(a, when=lambda x: x > 0)
a = -1  # Won't trigger
a = 1  # Trigger

When you do watch() on an object, you are actually tracking both the object and the variable holding it. In most cases, that's what you want anyways. However, you can configure precisely which you want to track.

a = []
watch(a, track="object")
a.append(1)  # Trigger
a = {}  # Won't trigger because the list object does not change

a = []
watch(a, track="variable")
a.append(1)  #  Won't trigger, because "a" still holds the same object
a = {}  # Trigger

Nested object comparison is tricky. It's hard to find a solid standard to compare complicated customized objects. By default, watchpoints will do a shallow copy of the object. You can override this behavior by passing deepcopy=True to watch()

watch(a, deepcopy=True)

watchpoints will honor __eq__ method for user-defined classes first. If __eq__ is not implemented, watchpoints will compare __dict__(basically attibures) of the object if using shallow copy, and raise an NotImplementedError if using deepcopy.

The reason behind this is, if you deepcopied a complicated structure, there's no way for watchpoints to figure out if it's the same object without user defined __eq__ function.

For your own data structures, you can provide a customized copy and/or customized compare function for watchpoints to better suit your need.

watchpoints will use the copy function you provide to copy the object for reference, and use your compare function to check if that object is changed. If copy function or compare function is not provided, it falls to default as mentioned above.

cmp argument takes a function that will take two objects as arguments and return a boolean representing whether the objects are different

def my_cmp(obj1, obj2):
    return obj1.id != obj2.id

watch(a, cmp=my_cmp)

copy argument takes a function that will take a object and return a copy of it

def my_copy(obj):
    return MyObj(id=obj.id)

watch(a, copy=my_copy)

You can specify the call stack limit printed using watch.config(). The default value is 5, any positive integer is accepted. You can use None for unlimited call stack, which means it will prints out all the frames.

watch.config(stack_limit=10)

You can also set different stack limits for each monitored variable by passing stack_limit argument to watch

# This will only change stack_limit for a
watch(a, stack_limit=10)

Of course sometimes you want to print in your own format, or even do something more than print. You can use your own callback for monitored variables

watch(a, callback=my_callback)

The callback function takes three arguments

def my_callback(frame, elem, exec_info)

• frame is the current frame when a change is detected.
• elem is a WatchElement object that I'm too lazy to describe for now.
• exec_info is a tuple of (funcname, filename, lineno) of the line that changed the variable

You can also set change the callback function globally by

watch.config(callback=my_callback)

Use restore() to restore the default callback

watch.restore()

watchpoints can be used with pdb with ease. You can trigger pdb just like using breakpoint() when your monitored variable is changed. Simply do

watch.config(pdb=True)

When you are in pdb, use q(uit) command to exit pdb, and the next change on the variable will trigger the pdb again.

Sometimes it's a hassle having to import the function in every single file. You can install the watch function to builtins and be able to call it in any files:

watch.install()  # or watch.install("func_name") and use it as func_name()
# Remove it from the builtins
watch.uninstall()  # if installed with a name, pass it to uninstall() as well

• watchpoints uses sys.settrace() so it is not compatible with other libraries that use the same function.
• watchpoints will slow down your program significantly, like other debuggers, so use it for debugging purpose only
• watch() needs to be used by itself, not nested in other functions, to be correctly parsed
• at this point, there might be other issues because it's still in development phase

Please send bug reports and feature requests through github issue tracker.

Copyright Tian Gao, 2020.

Distributed under the terms of the Apache 2.0 license.