Setting Breakpoints and Exception Hooks in Python


Final Up to date on January 22, 2022

There are alternative ways of debugging code in Python, one among which is to introduce breakpoints into the code at factors the place one want to invoke a Python debugger. The statements that one would use to enter a debugging session at totally different name websites, rely on the model of the Python interpreter that one is working with, as we will be seeing on this tutorial. 

On this tutorial, you’ll uncover numerous methods of setting breakpoints in numerous variations of Python. 

After finishing this tutorial, you’ll know:

  • Tips on how to invoke the pdb debugger in earlier variations of Python.
  • Tips on how to make use of the brand new, built-in
    breakpoint() operate launched in Python 3.7. 
  • Tips on how to write your individual
    breakpoint() operate to simplify the debugging course of in earlier variations of Python.
  • Tips on how to use a autopsy debugger

Let’s get began. 

Setting Breakpoints in Completely different Variations of Python
Photograph by Josh Withers, some rights reserved.

Tutorial overview

This tutorial is split into three elements; they’re:

  • Setting Breakpoints in Python Code
    • Invoking the pdb Debugger in Earlier Variations of Python
    • Utilizing the breakpoint() Operate in Python 3.7
  • Writing One’s Personal breakpoint() Operate for Earlier Variations of Python
  • Limitations of the breakpoint() operate

Setting breakpoints in Python code

We now have beforehand seen that a method of debugging a Python script is to run it within the command line with the Python debugger. 

So as to take action, we would wish to make use of of the
m pdb command that masses the pdb module earlier than executing the Python script. In the identical command line interface, we might then comply with this by a selected debugger command of selection, equivalent to
n to maneuver to the subsequent line, or
s if our intention is to step right into a operate. 

This technique may turn out to be shortly cumbersome because the size of the code will increase. One strategy to tackle this downside and acquire higher management over the place to interrupt your code, is to insert a breakpoint instantly into the code. 

Invoking the pdb debugger in earlier variations of Python

Doing so previous to Python 3.7 would require you to
import pdb, and to name
pdb.set_trace() on the level in your code the place you want to enter an interactive debugging session. 

If we rethink, for instance, the code for implementing the final consideration mechanism, we are able to break into the code as follows:

Executing the script now opens up the pdb debugger proper earlier than we compute the variable scores, and we are able to proceed to concern any debugger instructions of selection, equivalent to
n to maneuver to the subsequent line, or
c to proceed execution:

Though useful, this isn’t essentially the most elegant and intuitive strategy of inserting a breakpoint into your code. Python 3.7 implements a extra easy means of doing so, as we will see subsequent.

Utilizing the breakpoint() operate in Python 3.7 

Python 3.7 comes with a built-in
breakpoint() operate that enters the Python debugger on the name web site (or the purpose within the code at which the
breakpoint() assertion is positioned). 

When known as, the default implementation of the
breakpoint() operate will name
sys.breakpointhook(), which in flip calls the
pdb.set_trace() operate. That is handy as a result of we is not going to must
import pdb and name
pdb.set_trace() explicitly ourselves. 

Let’s rethink the code for implementing the final consideration mechanism, and now introduce a breakpoint through the
breakpoint() assertion:

One benefit of utilizing the
breakpoint() operate is that, in calling the default implementation of
sys.breakpointhook() the worth of a brand new atmosphere variable,
PYTHONBREAKPOINT, is consulted. This atmosphere variable can take numerous values, primarily based on which totally different operations might be carried out. 

For instance, setting the worth of
PYTHONBREAKPOINT to 0 disables all breakpoints. Therefore, your code may comprise as many breakpoints as crucial, however these might be simply stopped from halting the execution of the code with out having to take away them bodily. If (for instance) the identify of the script containing the code is, we might disable all breakpoints by calling it within the command line interface as follows:

In any other case, we are able to obtain the identical end result by setting the atmosphere variable within the code itself:

The worth of
PYTHONBREAKPOINT is consulted each time that
sys.breakpointhook() is named. Which means the worth of this atmosphere variable might be modified in the course of the code execution and the
breakpoint() operate would reply accordingly.  

PYTHONBREAKPOINT atmosphere variable may also be set to different values, equivalent to to the identify of a callable. Say, for example, that we’d like to make use of a special Python debugger apart from pdb, equivalent to ipdb (run
pip set up ipdb first, if the debugger has not but been put in). On this case, we might name the script within the command line interface, and hook the debugger with out making any modifications to the code itself:

In doing so, the
breakpoint() operate enters the ipdb debugger on the subsequent name web site:

The operate also can take enter arguments as,
breakpoint(*args, **kws), that are then handed on to
sys.breakpointhook(). It’s because any callable (equivalent to a 3rd occasion debugger module) would possibly settle for non-compulsory arguments, which might be handed by means of the
breakpoint() operate. 

Writing your individual breakpoint() operate in earlier variations of Python

Let’s return to the truth that variations of Python sooner than v3.7 don’t include the
breakpoint() operate readily inbuilt. We will write our personal. 

Equally to how the
breakpoint() operate is applied from Python 3.7 onwards, we are able to implement a operate that checks the worth of an atmosphere variable and:

  • Skips all breakpoints within the code if the worth of the atmosphere variable is ready to 0.
  • Enters into the default Python pdb debugger if the atmosphere variable is an empty string.
  • Enters into one other debugger as specified by the worth of the atmosphere variable. 

We will embrace this operate into the code and run it (utilizing a Python 2.7 interpreter, on this case). If we set the worth of the atmosphere variable to an empty string, we discover that the pdb debugger stops on the level within the code at which we’ve positioned our
breakpoint() operate. We will then concern debugger instructions into the command line from there onwards:

Equally, if we set the atmosphere variable to:

breakpoint() operate that we’ve applied now enters the ipdb debugger and stops on the name web site:

Setting the atmosphere variable to 0, merely skips all breakpoints and the computed consideration output is returned within the command line, as anticipated:

This facilitates the method of breaking into the code for Python variations sooner than v3.7, as a result of it now turns into a matter of setting the worth of an atmosphere variable, somewhat than having to manually introduce (or take away) the
import pdb; pdb.set_trace() assertion at totally different name websites within the code. 

Limitations of the breakpoint() operate

The breakpoint() operate permits you to carry within the debugger in the course of this system. You could discover the precise place that you just want the debugger to place the breakpoint into it. When you think about the next code:

it will carry you the debugger when the operate func() raised exceptions. It could actually triggered by the operate itself, or deep inside another capabilities that it calls. However the debugger will begin on the line print("exception!") above. Which will not be very helpful.

The way in which that we are able to carry up the debugger on the level of exception is named the autopsy debugger. It really works by asking Python to register the debugger because the exception handler when uncaught exception is raised. When it’s known as, it can search for the final exception raised and begin the debugger at that time. To make use of the autopsy debugger, we simply want so as to add the next code earlier than this system is run:

That is helpful as a result of nothing else have to be modified in this system. For example, assume we wish to consider the typical of $1/x$ utilizing the next program. It’s fairly straightforward to miss some nook instances however we are able to catch the difficulty when an exception is raised:

after we run the above program, this system might terminate or it might increase a division by zero exception, depends upon whether or not the random quantity generator ever produces zero within the loop. In that case, we might even see the next:

which we discovered the exception is raised at which line and we are able to examine the worth of the variables as we are able to normally do in pdb.

The truth is, it’s extra handy to print the traceback and the exception when the autopsy debugger is launched:

and the debugger session will likely be began as follows:

Additional Studying

This part supplies extra sources on the subject if you’re trying to go deeper.

Web sites


On this tutorial, you found numerous methods of setting breakpoints in numerous variations of Python. 

Particularly, you realized:

  • Tips on how to invoke the pdb debugger in earlier variations of Python. 
  • Tips on how to make use of the brand new, built-in
    breakpoint() operate launched in Python 3.7. 
  • Tips on how to write your individual
    breakpoint() operate to simplify the debugging course of in earlier variations of Python.

Do you’ve gotten any questions?

Ask your questions within the feedback under and I’ll do my finest to reply.