# GSoC Week 11: A major bug fix, and printing

Jim brought my attention to a very crucial bug in my work this week. Well, SymPy now automatically detects folders to test by looking at the __init__.py files. That is, a given folder in the SymPy code directory would _only_ be tested if it has __init__.py. Since I hadn’t put any such file in _sympy/vector/tests_, they were being skipped till now! Thank God this got fixed.

I started out this week with the seemingly ‘easy’ task of implementing different kinds of printing functionalities for sympy.vector. Well, the basic methodology of doing things is pretty simple, if you just follow the documentation in the _sympy.printing.printer_ file. It will tell you all you need to know about – either implementing a new printer, or adding custom code for printing of your class (to an existing printer).

Some things I had to improve/word around-

1. BaseScalars had to be handled differently from BaseVectors, since BaseScalars are a part of other Exprs (that may be measure numbers in vectorial expressions), and they have their own dedicated printing methods. Hence, I had to implement the _latex and _pretty methods in BaseScalar itself.

2. The pretty _as well as_ latex printing for _both_ Vectors and Dyadics with a single _print_BasisDependent method in the respective printers.

3. I used the printing code in _sympy.physics.vector_ as reference, though quite a few things had to be changed keeping in mind the inheritance from Expr, the structure and nature of the args being different, etc.
I did improve on a few things. _physics.vector_’s pretty printing messes up when dealing with vectors whose measure numbers have varying ‘heights’.

Overall, it was quite a learning experience and tricky coding – to implement printing, compared to my initial impression of it being a boring and tedious job.

Well, I have implemented all the requried methods and tests (all of which pass). I have pushed them to my third GSoC PR, so thats in line. Hopefully, the dyadics PR will soon show all tests passing, after which Jason or I will get it merged. It will add the OOP structure, and the dyadic code, as well as fix some crucial bugs.

A demo of the new printing implementations for a tedious vector expression-

Assume N is a CoordSysCartesian instance. Then,
(a**2 + b)*N.i + (Integral(f(b)))*N.k pretty-prints as

 u'\u239b 2 \u239e N_i + \u239b\u2320 \u239e N_k\n\u239da + b\u23a0 \u239c\u23ae f(b) db\u239f \n \u239d\u2321 \u23a0 '

and LaTeX-prints as-

 (a^{2} + b)\mathbf{\hat{i}_{N}} + (\int f{\left (b \right )}\, db)\mathbf{\hat{k}_{N}}