Add support for optional name validation of single-index

[jeffzi]

This PR adds optional name validation for single-index.

1. Previously, validation would fail on a named index if the index name was not set in the DataFrameSchema:

import pandas as pd
import pandera as pa

from pandera import Column, DataFrameSchema, Index, Check

schema = DataFrameSchema(index=Index(pa.Object))
df = pd.DataFrame(index=pd.Index(["index_1", "index_2", "index_3"], name="idx"))
schema.validate(df)
#> Traceback (most recent call last):
#> ...
#> SchemaError: Expected <class 'pandera.schema_components.Index'> to have name 'None', found 'idx'

^{Created on 2020-11-19 by the reprexpy package}

The new behavior is to disable name validation when the name is set to None. We discussed adding a new argument check_name to Index in #323 but I think this solution is more elegant. @cosmicBboy Let me know if you see any drawbacks to this approach.

Moreover, the PR does add a check_name parameters to Field. That closes #323

2. check_name (bool): Whether to check the name of the column/index during validation.
   None is the default behavior, which translates to True for columns and multi-index, and to False for a single index.

import pandera as pa
import pandas as pd

df = pd.DataFrame(index=pd.Index(["cat", "dog"], name="animal"))


class SchemaNamedIndex(pa.SchemaModel):
    a: pa.typing.Index[str]
    # Same as the following:
    # a: pa.typing.Index[str] = Field(check_name=False)
    # a: pa.typing.Index[str] = Field(check_name=None)


SchemaNamedIndex.validate(df)  # ok
#> Empty DataFrame
#> Columns: []
#> Index: [cat, dog]


class SchemaNamedIndex(pa.SchemaModel):
    a: pa.typing.Index[str] = pa.Field(check_name=True)


SchemaNamedIndex.validate(df)  # fails
#> Traceback (most recent call last):
#> ...
#> SchemaError: Expected <class 'pandera.schema_components.Index'> to have name 'a', found 'animal'

^{Created on 2020-11-19 by the reprexpy package}

3. As discussed here, columns must always be named.

4. Multi-index suffers from the same "bug" as described in 1. but it's much harder to fix because pa.MultiIndex is implemented as a subclass of pa.DataFrameSchema. I think it should be the topic of another issue.

import pandas as pd
import pandera as pa

from pandera import Column, DataFrameSchema, Index, MultiIndex, Check

schema = DataFrameSchema(index=MultiIndex([Index(pa.String), Index(pa.Int)]))
df = pd.DataFrame(
    index=pd.MultiIndex.from_arrays(
        [["foo", "bar", "foo"], [0, 1, 2]], names=["index0", "index1"]
    )
)
schema.validate(df)
#> Traceback (most recent call last):
#> ...
#> SchemaError: column '0' not in dataframe
#>               index0  index1
#> index0 index1               
#> foo    0         foo       0
#> bar    1         bar       1
#> foo    2         foo       2

^{Created on 2020-11-19 by the reprexpy package}

[codecov-io]

Codecov Report

Merging #326 (8cb53d3) into master (31fe07b) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #326      +/-   ##
==========================================
+ Coverage   98.77%   98.79%   +0.02%     
==========================================
  Files          18       18              
  Lines        1708     1747      +39     
==========================================
+ Hits         1687     1726      +39     
  Misses         21       21              

Impacted Files	Coverage Δ
pandera/model.py	100.00% <100.00%> (ø)
pandera/model_components.py	100.00% <100.00%> (ø)
pandera/schemas.py	98.12% <100.00%> (+0.14%)	⬆️

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[cosmicBboy]

👍 thanks @jeffzi!

[cosmicBboy]

The new behavior is to disable name validation when the name is set to None. We discussed adding a new argument check_name to Index in #323 but I think this solution is more elegant.

👍 💯