Develop Reference

Change code while debugging python program in Visual Studio Code

Is it possible to change the code while debugging in VSCode and that the change will take effect immediately without rerun the code?
I'm using Microsoft Python extension.

It depends. There's no such thing as a hot reload in Python. The closest you can come is importlib.reload(), but realize that only reloads the module and not the objects which already exist in memory. IOW it typically doesn't do what you want in code (it's usually used in the REPL).

So, apparently that the closest way of doing it is to use Jupyter capabilities (that can be use via VSCode).
With Jupyter you can split your code to multiple cells and run every each one of them separately without run all from beginning.

In Python, how do I debug with an interactive command line (and visual breakpoints?)

I've recently moved from Matlab to Python. Python is a much better language (from the point of view of a computer scientist), but Python IDEs all seem to lack one important thing:
A proper interactive debugger.
I'm looking for:
The ability to set breakpoints graphically by clicking next to a line of code in the editor.
The ability to run ANY CODE while stopped in the debugger, including calling functions from my code, showing new windows, playing audio, etc.
When an error occurs, the debugger should automatically open an interactive console at the error line.
Once done with the interactive console, you can resume normal execution.
Matlab has all these features and they work incredibly well, but I can't find them anywhere in Python tools.
I've tried:
PyCharm: the interactive console is clunky, often fails to appear, and crashes all the time (I've tried several different versions and OSs).
IPython: can't set breakpoints -Launching a Python console programatically: you have to stop your code, insert an extra line of code, and run again from the beginning to do this. Plus, you can't access functions already imported without re-importing them.
Being able to debug and fix problems THE FIRST TIME THEY APPEAR is very important to me, as I work in programs that often take dozens of minutes to re-run (computational neuroscience).
CONCLUSION: there is NO way to do all of these in Python at the moment. Let us hope that PyLab development accelerates.

At the top of your code, write
import pdb
And within your code, use the following statement wherever you want to debug.
pdb.set_trace()
You will have an interactive shell thus, whenever the set_trace() statement is met.
You can then use step(s), next(n), continue(c) and so on to check the execution flow, and print values of variables like print var
For more details on pdb, refer here

There are many Python IDEs. That was a topic here: What IDE to use for Python?
"The ability to set breakpoints graphically by clicking next to a line of code in the editor."
PyDev has this. Double-click in the gray margin bar.
"The ability to run ANY CODE while stopped in the debugger, including calling functions from my code, showing new windows, playing audio, etc."
PyDev has this. It's not the only one. PyScripter's stated features seem to include this.
"When an error occurs, the debugger should automatically open an interactive console at the error line."
PyDev does this. (I think. Or at worst do you need to double-click on the console message that states the error's location in the code?)
"Once done with the interactive console, you can resume normal execution."
PyDev has this. It's called "resume". It's what the green "play" triangle in a toolbar does. Some other software calls this feature "continue".

I've been searching for the same, but unfortunately Python IDEs are not as well-featured as Matlab's at this point. For scientific programming, you will also want graphics/plotting to run in an entirely different thread, so IPython integration is essential. As far as I can tell, the Matlab IDE feature to change the workspace from the debugger, which then affects code running subsequently, is quite unique. Each of the features exist in some IDE, but none exist in all:
Spyder has good integration with scientific tools, but its debugging is limited to the built-in pdb, which lacks the requirement to execute any code and have this code affect the namespace after continuing.
PyDev and PyCharm, and quite a few others, have decent debugging features, but I don't think it has good integration with scientific tools. That means, if you plot, you lose access to your prompt. Not good.
As far as I've experienced, the closest comes Wing IDE. It is a propietry product, but if you're making the transition from Matlab 89$/year for non-commercial-use should be acceptable (you can evaluate it first). But for me, I've ultimately settled to alter my workflow, and not using any sophisticated IDE at all. When I looked was some years ago, so perhaps the situation has improved.
You might also be interested in this article from April 2013, Evaluating IDEs for Scientific Python. It doesn't really reach a conclusion either.

Seeing as you are comming from Matlab, I would suggest you give a look at
Python(x,y)
The page decribes it as follows:
Python(x,y) is a free scientific and engineering development software for numerical computations, data analysis and data visualization based on Python programming language, Qt graphical user interfaces and Spyder interactive scientific development environment.
It will not cater to all your wishes, but it certainly made me feel comfortable when I started out with Python, coming from Matlab.
Hope it helps

You can do all this in the iPython Notebook. Use the magic command %pdb to stop on error.

If using the command line,
alias ipythondebug='ipython --InteractiveShell.pdb true'
in your ~/.profile will give you debug on error like Matlab. This of course requires ipython installed.
Not sure about the resuming part.
You can also edit the ipython config file if you want the debug on error to be permanent. See
https://ipython.readthedocs.io/en/stable/interactive/magics.html#magic-pdb

Debug crashing C Library used in Python project

complete Python noob here (and rusty in C).
I am using a Mac with Lion OS. Trying to use NFCpy, which uses USBpy, which uses libUSB. libUSB is crashing due to a null pointer but I have no idea how to debug that since there are so many parts involved.
Right now I am using xcode to view the code highlighted but I run everything from bash. I can switch to Windows or Linux if this is going to be somehow easier with a different environment.
Any suggestions on how to debug this would be much appreciated ;-)
PS: It would be just fine if I could see the prints I put in C in the bash where I run the Python script

You should see your printf() you put in C in your terminal, something is already wrong here. Are you sure that you're using the latest compiled library? To be sure, instead of print, you can use use assert(0) (you need to include assert.h).
Anyway, you can debug your software using gdb:
gdb --args python yourfile.py
# type "run" to start the program
# if you put assert() in your code, gdb will stop at the assert, or you can put
# manual breakpoint by using "b filename:lineno" before "run"

Enable core dumps (ulimit -Sc unlimited) and crash the program to produce a core file. Examine the core file with gdb to learn more about the conditions leading up to the crash. Inspect the functions and local variables on the call stack for clues.
Or run the program under gdb to begin with and inspect the live process after it crashes and gdb intercepts the signal (SIGSEGV, SIGBUS, whatever).
Both of these approaches will be easier if you make sure all relevant native code (Python, libUSB, etc) have debugging symbols available.
Isolating the problem in a program which is as small as you can manage to make it, as Tio suggested, will also make this process easier.
PS: It would be just fine if I could see the prints I put in C in the bash where I run the Python script
You didn't mention anything about adding prints "in C" elsewhere in your question. Did you modify libUSB to add debugging prints? If so, did you rebuild it? What steps did you take to ensure that your new build would be used instead of the previously available libUSB? You may need to adjust your dylib-related environment variables to get the dynamic linker to prefer your version over the system version. If you did something else, explain what. :)

Experienced Python programmers (ESPECIALLY former php programmers) : How do I debug python?

In PHP, it was extremely easy to start hacking away and figuring out what was going on on a page. Just throw in a bunch of echos and print_r's and that was about it. It appears that this technique is not working for me in python. I am getting practice by hacking around in a python photo upload module, and when a photo is uploaded, it creates 3 different size photos. I found the code that does this, but I want to see the state at that particular moment. I tried doing a "print" on the size variable, but it did not show up in my browser.
I guess a more straightforward question would be, is it "pythonic" do debug using the browser ( equivalent to echo's and print_r's in php ), or is this what the python console is for? Thanks!

Use the logging module rather than printing stuff to stdout.
Using the interpreter in interactive mode is a great way to try out code, and pdb is very useful for real debugging.

It's "pythonic" to do debugging using the pdb module.
But really, if you're just "hacking around", then I suggest messing around with an interactive interpreter interface, especially one that supports autocompletion (Python itself comes with IDLE right out of the box).

You need to learn how to use a debugger ;) Hacking with prints is cool for simple php, but you can save a lot of time in higher languages with a debugger.
As mentioned, PDP is a place to start http://docs.python.org/library/pdb.html

As others have mentioned PDB, I'll take the opportunity to sing the praises of Eclipse using the pydev plugin, which is absolutely fantastic. IDLE is also well worth a go. Both of these IDEs allow you to step through code, inspect variables, auto-complete, etc. etc.
http://pydev.org/
Download Eclipse then use the Software Updates menu to add in PyDev.
http://pydev.org/updates

PyCharm from JetBrains is as good as all their other products. It has an integrated debugger and lots more.

Easiest way to pop up interactive Python console?

My application has a Python interpreter embedded in it. However there's currently not any way to inspect anything about Python directly. I'd like to be able to pop up an interactive shell at various points to inspect what's going on in Python.
I've found several similar questions which pointed me to code.InteractiveConsole and IPython. However neither of those seem to do anything when I call them - the call appears to complete okay, but nothing visible happens, presumably because my Python instance isn't outputting anywhere I can see.
It was pretty optimistic to think that they'd just magically pop up a new window for me, but it would be nice if I could get that to happen. So, does anybody know if there is an easy way to achieve this (or something similar), or do I have to roll my own dialog and pass the input to Python and display output myself?
Thanks :)

What have you tried with IPython? Is it the snippets from the documentation:
Embedding IPython (IPython docs)
How about some of the code samples from elsewhere:
Embedding IPython in GUI apps is trivial
Embedding IPython in a PyGTK application
I know I fooled around with this a while back and the samples seemed to work, but then I never got a chance to go any further for other reasons.

I know this isn't what you're asking, but when I've wanted to debug compiled Python (using Py2Exe), I've been very pleased to realize that I can add breakpoints to the exe and it will actually stop there when I start the executable from a console window. Simply add:
import pdb
pdb.set_trace()
where you want your code to stop, and you'll have yourself an interactive debugging session with a compiled executable. Python is awesome that way :)