PythonWise

If it won't be simple, it simply won't be. [source code] by Miki Tebeka, CEO, 353Solutions

Tuesday, August 30, 2016

"Manual" Breakpoints in Go

When debugging, sometimes you need to set conditional breakpoints. This option is available both in gdb and delve. However sometimes when the condition is complicated, it's hard or even impossible to set it. A way around is to temporary write the condition in Go and set breakpoint "manually".

I Python we do it with pdb.set_trace(), in Go we'll need to work a little harder. The main idea is that breakpoints are special signal called SIGTRAP.

Here's the code to do this:
You'll need tell the go tool not to optimize and keep variable information:

$ go build -gcflags "-N -l" manual-bp

Then run a gdb session

$ gdb manual-bp 
(gdb) run 

 When you hit the breakpoint, you'll be in assembly code. Exit two functions to get to your code

(gdb) fin
(gdb) fin

Then you'll be in your code and can run gdb commands

(gdb) p i
$1 = 3

This scheme also works with delve

$ dlv debug manual-bp.go 
(dlv) c 

Sadly delve don't have "fin" command so you'll need to hit "n" (next) until you reach your code. 

That's it, happy debugging.

Oh - and in the very old days we did about the same trick in C code. There we manually inserted asm("int $3)" to the code. You can do with with cgo but sending a signal seems easier.

Wednesday, August 24, 2016

Generate Relation Diagram from GAE ndb Model

Working with GAE, we wanted to create relation diagram from out ndb model. By deferring the rendering to dot and using Python's reflection this became an easy task. Some links are still missing since we're using ancestor queries, but this can be handled by some class docstring syntax or just manually editing the resulting dot file.

Friday, June 10, 2016

Work with AppEngine SDK in the REPL

Working again with AppEngine for Python. Here's a small code snippet that will let you work with your code in the REPL (much better than the previous solution).
What I do in IPython is:
In [1]: %run initgae.py

In [2]: %run app.py

And then I can work with my code and test things out.

Monday, May 30, 2016

Using ImageMagick to Generate Images

One of the exercises we did this week in the Python workshop used the term "bounding box diagonal". I had a hard time to explain it to the students without an image. Google image search didn't find anything great, so I decided to create such an image.

First I tried with drawing programs, but couldn't make the rectangle a square and the circle non-oval. Then I remembered imagemagick, I have it installed and mostly use it to resize images - but it can do much more. A quick look at the examples and some trial and error, and here's the result.


And here's the script that generated it:

Saturday, April 16, 2016

Waiting for HTTP Server - Go Testing

I don't like mocking in tests. If I have a server to test, I prefer to start and instance and hit the API in my tests. Waiting for the server to start with a simple sleep is unpredictable. I prefer to start the server, try to hit an URL until it's OK or fail after a long timeout. This is a simple task with Go's select and time.After.

Tuesday, March 29, 2016

Slap a --help on it

Sometimes we write "one off" scripts to deal with certain task. However most often than not these scripts live more than just the one time. This is very common in ops related code that for some reason people don't apply the regular coding standards to.

It really upsets me when I try to see what a script is doing, run it with --help flag and it happily deletes the database while I wait :) It's so easy to add help support in the command line. In Python we do it with argparse, and we role our own in bash. Both cases it's extra 3 lines of code.

Please be kind to future self and add --help support to your scripts.

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