Python breakpoint() function

Python breakpoint() is a new built-in function introduced in Python 3.7. Python code debugging has always been a painful process because of the tight coupling between the actual code and the debugging module code. For example, if you are using pdb debugger, then you will have to call pdb.set_trace() in your program code. If you want to use any other debugger, let’s say web-pdb then you will have to remove all the code related to PDB and add web_pdb.set_trace() method. This causes a huge overhead in using python debugger and makes the python code hard to debug and maintain. That’s why Python 3.7 has introduced breakpoint() method that allows us to write loosely coupled debugging code.

How Python breakpoint() Works

Python breakpoint() function calls sys.breakpointhook() function. By default, sys.breakpointhook() calls pdb.set_trace() function. So at the very least, using breakpoint() provide convenience in using a debugger because we don’t have to explicitly import pdb module. Let’s look at a simple example of breakpoint() function usage. We have a python script python_breakpoint_examples.py with the following code.

x = 10
y = 'Hi'
z = 'Hello'
print(y)

breakpoint()

print(z)

When we execute this script, the PDB debugger console opens up.

$python3.7 python_breakpoint_examples.py
Hi
> /Users/pankaj/Documents/PycharmProjects/BasicPython/basic_examples/python_breakpoint_examples.py(8)()
-> print(z)
(Pdb) c
Hello
$

Stopping Debugging with Python breakpoint()

Python sys.breakpointhook() function uses environment variable PYTHONBREAKPOINT to configure the debugger. If unset, the default PDB debugger is used. If it’s set to “0” then the function returns immediately and no code debugging is performed. It’s very helpful when we want to run our code without debugging.

$PYTHONBREAKPOINT=0 python3.7 python_breakpoint_examples.py
Hi
Hello
$

Changing Debugger Module with Python breakpoint()

We can use PYTHONBREAKPOINT environment variable to provide the debugger method to be called by breakpoint() function. This is very helpful because we can change the debugger module easily without making any code change. Let’s say we want to use web-pdb debugger. We can easily hook it into our program using PYTHONBREAKPOINT=web_pdb.set_trace. First of all, make sure that web-pdb is installed. You can install it using pip3.7 install web-pdb command.

$PYTHONBREAKPOINT=web_pdb.set_trace python3.7 python_breakpoint_examples.py
Hi
2018-08-10 12:49:54,339: root - web_console:110 - CRITICAL - Web-PDB: starting web-server on pankaj:5555...

Open the web-server URL provided in the console log and you will see the debugger window. We can issue PDB commands using this UI, you can send command “c” to continue and complete our program.

Summary

Python breakpoint() function is a very helpful addition to the python debugging feature. It’s recommended to use this for debugging so that you can easily hook other third-party debuggers on the fly. It also provides an easy option to disable the debugger and runs the program normally.