adds explanatory error message to check decorators and tests

[mastersplinter]

This PR:

• adds a detailed explanation when the user specifies an invalid index to a check decorator
• this is particularly useful in a long/complex pipeline undergoing refactoring

In this example:

import pandas as pd
from pandera import Column, Float, DataFrameSchema, check_input, check_output

df_schema = DataFrameSchema(
        {"a":        Column(Float, nullable=True),
         "b":        Column(Float, nullable=True),
         })

df1 = pd.DataFrame({'a':[1.,2.], 'b':[1.,2.]})
df2 = pd.DataFrame({'a':[1.,2.], 'b':[1.,2.]})

@check_input(df_schema, 2)
@check_output(df_schema)
def testing_func(a,b):
    out = a.append(b)
    return out

testing_func(df1,df2)

before PR:

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-2-ddac0478d816> in <module>
     16     return out
     17 
---> 18 testing_func(df1,df2)

/pandera_dev/pandera/pandera.py in _wrapper(fn, instance, args, kwargs)
   1004         args = list(args)
   1005         if isinstance(obj_getter, int):
-> 1006             args[obj_getter] = schema.validate(args[obj_getter])
   1007         elif isinstance(obj_getter, str):
   1008             if obj_getter in kwargs:

IndexError: list index out of range

after the PR the error is:

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
/pandera_dev/pandera/pandera.py in _wrapper(fn, instance, args, kwargs)
   1006             try:
-> 1007                 args[obj_getter] = schema.validate(args[obj_getter])
   1008             except IndexError as e:

IndexError: list index out of range

During handling of the above exception, another exception occurred:

SchemaError                               Traceback (most recent call last)
<ipython-input-2-ddac0478d816> in <module>
     16     return out
     17 
---> 18 testing_func(df1,df2)

/pandera_dev/pandera/pandera.py in _wrapper(fn, instance, args, kwargs)
   1016                          len(_get_fn_argnames(fn)),
   1017                          max(0, len(_get_fn_argnames(fn))-1),
-> 1018                          e
   1019                          )
   1020                         )

SchemaError: error in check decorator of function 'testing_func': the index '2' was supplied to the check but this function accepts '2' arguments, so the maximum index is '1'. The full error is: 'list index out of range'

[mastersplinter]

@cosmicBboy this is now ready for re-review, apologies for the new PR, I squashed the history to ensure it was tidy for the merge and git won't open a PR when the branch has been force-pushed.

[cosmicBboy]

approved! left one suggestion

[codecov-io]

Codecov Report

Merging #86 into master will increase coverage by 0.12%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #86      +/-   ##
==========================================
+ Coverage   92.83%   92.96%   +0.12%     
==========================================
  Files           2        2              
  Lines         335      341       +6     
==========================================
+ Hits          311      317       +6     
  Misses         24       24

Impacted Files	Coverage Δ
pandera/pandera.py	92.92% <100%> (+0.12%)	⬆️

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