Return an informative error message when spec should be a list

[hinnefe2]

glom is great, thanks for making it!

This PR makes the error message more informative when your target is a list and your spec is not.

Currently when target is a list but spec is not you get

ValueError: invalid literal for int() with base 10:

which doesn't immediately suggest that you forgot to make your spec a list. I've run into this a couple times now, and each time I forgot about last time and ended up having to check the source code to figure out what was going on. Next time I might finally remember, but for other people hitting this a different error message might help. With this change, the error message is

ValueError: Error indexing the target. This can happen when your target is a list and your spec is not. Did you forget to put brackets around your spec?

Please let me know if this check makes more sense somewhere else, I just put it right where the error first gets raised.

Thanks!

MWE:

from glom import glom

target = [{'xxx': i} for i in range(3)]

spec = {'number': 'xxx'}

glom(target, spec)

[codecov-io]

Codecov Report

Merging #62 into master will decrease coverage by 0.11%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master      #62      +/-   ##
==========================================
- Coverage   94.45%   94.34%   -0.12%     
==========================================
  Files          14       14              
  Lines        1479     1485       +6     
  Branches      231      232       +1     
==========================================
+ Hits         1397     1401       +4     
- Misses         47       48       +1     
- Partials       35       36       +1

Impacted Files	Coverage Δ
glom/core.py	96.48% <71.42%> (-0.27%)	⬇️

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[kurtbrose]

hey this looks great, thanks!

[mahmoud]

Hey, thanks! I agree with Kurt, this is indeed great. I've been iterating on the idea of having a top-level exception raised to simplify the stack and make the error more actionable. I think this definitely falls into that vein. I'll need to think a bit more about this exception message-based approach. Might this make the behavior a bit version-specific?

[hinnefe2]

Hey - sorry for the delayed response! I see what you mean about relying on the exception message, I'm not sure how stable those messages are across python versions.

What about an explicit check on whether int(index) raises a ValueError? That catches the problem I was running into and seems narrow enough not to mask anything else.