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diff --git a/blog/2022-05-25-using-pdb.gmi b/blog/2022-05-25-using-pdb.gmi new file mode 100644 index 0000000..6d9562a --- /dev/null +++ b/blog/2022-05-25-using-pdb.gmi @@ -0,0 +1,109 @@ +# Using PDB to Troubleshoot Python Code + +# Tags +* date: 2022-05-17T00:00:00+03:00 +* draft: false +* tags: debugging, python +* author: bonfacem + + +Historically, debugging GeneNetwork code has been a bit of a pain where you would have `print' and `logging' statements to help view offending code chunks. This is not efficient, and we can do better! One painful side-effect wrt logging worth mentioning is that our logs grow quite fast and we need to rotate them, atm manually: + +Here are examples of some logging that we do: + +``` +@app.route("/n/logout") +def logout(): + logger.debug("Logging out...") + UserSession().delete_session() + flash("You are now logged out. We hope you come back soon!") + response = make_response(redirect(url_for('index_page'))) + # Delete the cookie + response.set_cookie(UserSession.user_cookie_name, '', expires=0) + return response +``` + +``` +@app.route("/tmp/<img_path>") def +tmp_page(img_path): logger.info("In tmp_page") +logger.info("img_path:", img_path) +logger.info(request.url) initial_start_vars = +request.form logger.info("initial_start_vars:", +initial_start_vars) imgfile = +open(GENERATED_IMAGE_DIR + img_path, 'rb') imgdata += imgfile.read() imgB64 = +base64.b64encode(imgdata) bytesarray = +array.array('B', imgB64) return +render_template("show_image.html", +img_base64=bytesarray) +``` + +Earlier this year, one of our members introduced us to pudb---a graphical based logging utility for Python. I have gravitated away from this because it adds yet another dependency in our toolchain; in addition to it being ncurses-based, lacking guarantees in how it behaves in different terminals. It also lacks support in different OS'es, thereby forcing end-users to SSH into one of our remote servers to troubleshoot. + +Python PDB ships with Python, and as such, works well in different setups. There are multiple ways of getting into a pdb session, the easiest being to set a `breakpoint()'. Assume we are trouble-shooting this function: + +``` +from typing import List + + +def avg(numbers: List) -> int: + return sum(numbers)/len(numbers) + + +print(avg([20, 21])) +``` + +This will fail for a list that contains non-integer value, say a list containing ["1", "2"]. The first step to troubleshoot, assuming we have no test would be to set a `breakpoint()' as such: + +``` +from typing import List + + +def avg(numbers: List) -> int: + breakpoint() + return sum(numbers)/len(numbers) + + +print(avg([20, "21"])) +``` + +Useful commands while you are in pdb that are useful: + +* "l ." --- show where you are in the context +* "n" --- go to the next line +* "s" --- step into a function +* "c" --- continues execution until a breakpoint is encountered +* "p" --- print a variable +* "pp" --- pretty print a variable + +When we step into our debug session, we can view all the variables in a local scope using: "locals()"; and the global scope using: "globals()". With this information, we can quickly work out where our problem is by just inspecting the variables we have at hand. + +Another cool trick/pattern when debugging is to tell pdb to jump to where the error occured in a try/except block using `import pdb; pdb.post_mortem()' like so: + +``` +from typing import List + + +def avg(numbers: List) -> int: + try: + return sum(numbers)/len(numbers) + except Exception: + import pdb; pdb.post_mortem() + + +print(avg([20, "21"])) +``` + +With regards to testing, pdb is also integrated with test-runners. To use pdb with pytest, simply run: + +,---- +| pytest --pdb +`---- + + +### Useful Tutorials + +To learn more about pdb, you can check out: + +=> https://www.youtube.com/watch?v=0LPuG825eAk python debugger crash course: pdb/breakpoint +=> https://www.youtube.com/watch?v=s8Nx2frW4ps postmortem debugging in python |