How to step through Python code to help debug issues?

Question

In Java/C# you can easily step through code to trace what might be going wrong, and IDE's make this process very user friendly.

Can you trace through python code in a similar fashion?

Answer 1

Yes! There's a Python debugger called pdb just for doing that!

You can launch a Python program through pdb via python -m pdb myscript.py.

There are a few commands you can then issue, which are documented on the pdb page.

Some useful ones to remember are:

• b: set a breakpoint
• c: continue debugging until you hit a breakpoint
• s: step through the code
• n: to go to next line of code
• l: list source code for the current file (default: 11 lines including the line being executed)
• u: navigate up a stack frame
• d: navigate down a stack frame
• p: to print the value of an expression in the current context

If you don't want to use a command line debugger, some IDEs like Pydev, Wing IDE or PyCharm have a GUI debugger. Wing and PyCharm are commercial products, but Wing has a free "Personal" edition, and PyCharm has a free community edition.

Answer 2

By using Python Interactive Debugger 'pdb'

First step is to make the Python interpreter enter into the debugging mode.

A. From the Command Line

Most straight forward way, running from command line, of python interpreter

$ python -m pdb scriptName.py
> .../pdb_script.py(7)<module>()
-> """
(Pdb)

B. Within the Interpreter

While developing early versions of modules and to experiment it more iteratively.

$ python
Python 2.7 (r27:82508, Jul  3 2010, 21:12:11)
[GCC 4.0.1 (Apple Inc. build 5493)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import pdb_script
>>> import pdb
>>> pdb.run('pdb_script.MyObj(5).go()')
> <string>(1)<module>()
(Pdb)

C. From Within Your Program

For a big project and long-running module, can start the debugging from inside the program using import pdb and set_trace() like this:

#!/usr/bin/env python
# encoding: utf-8
#

import pdb

class MyObj(object):
    count = 5
    def __init__(self):
        self.count= 9

    def go(self):
        for i in range(self.count):
            pdb.set_trace()
            print i
        return

if __name__ == '__main__':
    MyObj(5).go()

Step-by-Step debugging to go into more internal

1. Execute the next statement… with “n” (next)

2. Repeating the last debugging command… with ENTER

3. Quitting it all… with “q” (quit)

4. Printing the value of variables… with “p” (print)

   a) p a

5. Turning off the (Pdb) prompt… with “c” (continue)

6. Seeing where you are… with “l” (list)

7. Stepping into subroutines… with “s” (step into)

8. Continuing… but just to the end of the current subroutine… with “r” (return)

9. Assign a new value

   a) !b = "B"

10. Set a breakpoint

    a) break linenumber

    b) break functionname

    c) break filename:linenumber

11. Temporary breakpoint

    a) tbreak linenumber

12. Conditional breakpoint

    a) break linenumber, condition

Note: All these commands should be executed from pdb

For in-depth knowledge, refer:

• https://pymotw.com/2/pdb/

• https://pythonconquerstheuniverse.wordpress.com/2009/09/10/debugging-in-python/

Answer 3

There is a module called 'pdb' in python. At the top of your python script you do

import pdb
pdb.set_trace()

and you will enter into debugging mode. You can use 's' to step, 'n' to follow next line similar to what you would do with 'gdb' debugger.

Answer 4

Starting in Python 3.7, you can use the breakpoint() built-in function to enter the debugger:

foo()
breakpoint()  # drop into the debugger at this point
bar()

By default, breakpoint() will import pdb and call pdb.set_trace(). However, you can control debugging behavior via sys.breakpointhook() and use of the environment variable PYTHONBREAKPOINT.

See PEP 553 for more information.

Answer 5

ipdb (IPython debugger)

ipdb adds IPython functionality to pdb, offering the following HUGE improvements:

• tab completion
• show more context lines
• syntax highlight

Much like pdg, ipdb is still far from perfect and completely rudimentary if compared to GDB, but it is already a huge improvement over pdb.

Usage is analogous to pdb, just install it with:

python3 -m pip install --user ipdb

and then add to the line you want to step debug from:

__import__('ipdb').set_trace(context=21)

You likely want to add a shortcut for that from your editor, e.g. for Vim snipmate I have:

snippet ipd
    __import__('ipdb').set_trace(context=21)

so I can type just ipd<tab> and it expands to the breakpoint. Then removing it is easy with dd since everything is contained in a single line.

context=21 increases the number of context lines as explained at: How can I make ipdb show more lines of context while debugging?

Alternatively, you can also debug programs from the start with:

ipdb3 main.py

but you generally don't want to do that because:

• you would have to go through all function and class definitions as Python reads those lines
• I don't know how to set the context size there without hacking ipdb. Patch to allow it: https://github.com/gotcha/ipdb/pull/155

Or alternatively, as in raw pdb 3.2+ you can set some breakpoints from the command line:

ipdb3 -c 'b 12' -c 'b myfunc' ~/test/a.py

although -c c is broken for some reason: https://github.com/gotcha/ipdb/issues/156

python -m module debugging has been asked at: How to debug a Python module run with python -m from the command line? and since Python 3.7 can be done with:

python -m pdb -m my_module

Serious missing features of both pdb and ipdb compared to GDB:

• persistent command history across sessions: Save command history in pdb

ipdb specific annoyances:

• multithreading does not work well if you don't hack some settings...
  • ipdb, multiple threads and autoreloading programs causing ProgrammingError
  • https://github.com/gotcha/ipdb/issues/51

Tested in Ubuntu 16.04, ipdb==0.11, Python 3.5.2.

Answer 6

VSCode

If you want to use an IDE, this is a good alternative to PyCharm.

1. Install VSCode
2. Install the Python extension, if it's not already installed
3. For example, create a file mymodule.py with Python code
4. To set a breakpoint, hover over a line number and click the red dot, or press F9
5. Hit F5 to start debugging and select Python File

It will stop at the breakpoint and you can do your usual debugging stuff like inspecting the values of variables, either at the tab VARIABLES (usually on the left) or by clicking on Debug Console (usually at the bottom next to your Terminal):

This screenshot shows VSCodium.

More information

• Python debugging in VS Code
• Getting Started with Python in VS Code
• Debugging in Visual Studio Code

Answer 7

There exist breakpoint() method nowadays, which replaces import pdb; pdb.set_trace().

It also has several new features, such as possible environment variables.

Answer 8

Python Tutor is an online single-step debugger meant for novices. You can put in code on the edit page then click "Visualize Execution" to start it running.

Among other things, it supports:

• hiding variables, e.g. to hide a variable named x, put this at the end:

  #pythontutor_hide: x
  

• saving/sharing

• a few other languages like Java, JS, Ruby, C, C++

However it also doesn't support a lot of things, for example:

• Reading/writing files - use io.StringIO and io.BytesIO instead: demo
• Code that is too large, runs too long, or defines too many variables or objects
• Command-line arguments
• Lots of standard library modules like argparse, csv, enum, html, os, sys, weakref...
• Python 3.7+

Answer 9

Let's take look at what breakpoint() can do for you in 3.7+.

I have installed ipdb and pdbpp, which are both enhanced debuggers, via

pip install pdbpp
pip install ipdb

My test script, really doesn't do much, just calls breakpoint().

#test_188_breakpoint.py
myvars=dict(foo="bar")
print("before breakpoint()")
breakpoint()   # 
print(f"after breakpoint myvars={myvars}")

breakpoint() is linked to the PYTHONBREAKPOINT environment variable.

CASE 1: disabling breakpoint()

You can set the variable via bash as usual

export PYTHONBREAKPOINT=0

This turns off breakpoint() where it does nothing (as long as you haven't modified sys.breakpointhook() which is outside of the scope of this answer).

This is what a run of the program looks like:

(venv38) myuser@explore$ export PYTHONBREAKPOINT=0
(venv38) myuser@explore$ python test_188_breakpoint.py
before breakpoint()
after breakpoint myvars={'foo': 'bar'}
(venv38) myuser@explore$

Didn't stop, because I disabled breakpoint. Something that pdb.set_trace() can't do !

CASE 2: using the default pdb behavior:

Now, let's unset PYTHONBREAKPOINT which puts us back to normal, enabled-breakpoint behavior (it's only disabled when 0 not when empty).

(venv38) myuser@explore$ unset PYTHONBREAKPOINT
(venv38) myuser@explore$ python test_188_breakpoint.py
before breakpoint()
[0] > /Users/myuser/kds2/wk/explore/test_188_breakpoint.py(6)<module>()
-> print(f"after breakpoint myvars={myvars}")
(Pdb++) print("pdbpp replaces pdb because it was installed")
pdbpp replaces pdb because it was installed
(Pdb++) c
after breakpoint myvars={'foo': 'bar'}

It stopped, but I actually got pdbpp because it replaces pdb entirely while installed. If I unistalled pdbpp, I'd be back to normal pdb.

Note: a standard pdb.set_trace() would still get me pdbpp

CASE 3: calling a custom debugger

But let's call ipdb instead. This time, instead of setting the environment variable, we can use bash to set it only for this one command.

(venv38) myuser@explore$ PYTHONBREAKPOINT=ipdb.set_trace py test_188_breakpoint.py
before breakpoint()
> /Users/myuser/kds2/wk/explore/test_188_breakpoint.py(6)<module>()
      5 breakpoint()
----> 6 print(f"after breakpoint myvars={myvars}")
      7

ipdb> print("and now I invoked ipdb instead")
and now I invoked ipdb instead
ipdb> c
after breakpoint myvars={'foo': 'bar'}

Essentially, what it does, when looking at $PYTHONBREAKPOINT:

from ipdb import set_trace  # function imported on the right-most `.`
set_trace()

Again, much cleverer than a plain old pdb.set_trace()

in practice? I'd probably settle on a debugger.

Say I want ipdb always, I would:

• export it via .profile or similar.
• disable on a command by command basis, without modifying the normal value

Example (pytest and debuggers often make for unhappy couples):

(venv38) myuser@explore$ export PYTHONBREAKPOINT=ipdb.set_trace
(venv38) myuser@explore$ echo $PYTHONBREAKPOINT
ipdb.set_trace
(venv38) myuser@explore$ PYTHONBREAKPOINT=0 pytest test_188_breakpoint.py
=================================== test session starts ====================================
platform darwin -- Python 3.8.6, pytest-5.1.2, py-1.9.0, pluggy-0.13.1
rootdir: /Users/myuser/kds2/wk/explore
plugins: celery-4.4.7, cov-2.10.0
collected 0 items

================================== no tests ran in 0.03s ===================================
(venv38) myuser@explore$ echo $PYTHONBREAKPOINT
ipdb.set_trace

p.s.

I'm using bash under macos, any posix shell will behave substantially the same. Windows, either powershell or DOS, may have different capabilities, especially around PYTHONBREAKPOINT=<some value> <some command> to set a environment variable only for one command.

Answer 10

https://wiki.python.org/moin/PythonDebuggingTools

pudb is a good drop-in replacement for pdb

Answer 11

If you come from Java/C# background I guess your best bet would be to use Eclipse with Pydev. This gives you a fully functional IDE with debugger built in. I use it with django as well.

Answer 12

PyCharm is an IDE for Python that includes a debugger. Watch this YouTube video for an introduction on using it to step through code:

PyCharm Tutorial - Debug python code using PyCharm (the debugging starts at 6:34)

Note: PyCharm is a commercial product, but the company does provide a free license to students and teachers, as well as a "lightweight" Community version that is free and open-source.

Answer 13

Programmatically stepping and tracing through python code is possible too (and its easy!). Look at the sys.settrace() documentation for more details. Also here is a tutorial to get you started.

Answer 14

Visual Studio with PTVS could be an option for you: http://www.hanselman.com/blog/OneOfMicrosoftsBestKeptSecretsPythonToolsForVisualStudioPTVS.aspx