Fixturize Test Excel
Continued simplification of this module by moving towards pytest idiom. Here I have eliminated any test instance methods and replaced with fixtures 

Hello @WillAyd! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:



There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2019-05-29 16:39:12 UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26543?src=pr&el=h1) Report
> Merging [#26543](https://codecov.io/gh/pandas-dev/pandas/pull/26543?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/998a0deea39f11fa06071af77cc1afba65900330?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26543/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26543?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26543      +/-   ##
==========================================
- Coverage   91.77%   91.76%   -0.01%     
==========================================
  Files         174      174              
  Lines       50638    50638              
==========================================
- Hits        46471    46467       -4     
- Misses       4167     4171       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.3% <ø> (ø)` | :arrow_up: |
| #single | `41.67% <ø> (-0.09%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26543?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26543/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26543/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26543?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26543?src=pr&el=footer). Last update [998a0de...fb79318](https://codecov.io/gh/pandas-dev/pandas/pull/26543?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26543?src=pr&el=h1) Report
> Merging [#26543](https://codecov.io/gh/pandas-dev/pandas/pull/26543?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/a91da0c94e541217865cdf52b9f6ea694f0493d3?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26543/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26543?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26543      +/-   ##
==========================================
- Coverage   91.77%   91.76%   -0.01%     
==========================================
  Files         174      174              
  Lines       50649    50649              
==========================================
- Hits        46483    46479       -4     
- Misses       4166     4170       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.3% <ø> (ø)` | :arrow_up: |
| #single | `41.69% <ø> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26543?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26543/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26543/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26543?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26543?src=pr&el=footer). Last update [a91da0c...3a5e5bb](https://codecov.io/gh/pandas-dev/pandas/pull/26543?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @simonjayhawkins I think looking a lot better after the monkeypatch. Let me know what you think > is the engine parameter of read_excel ignored when using ExcelFile?

Yes ExcelFile has it's own engine parameter. If that's passed to read_excel along with another engine argument it should probably raise or warn. I'll open that as a follow up See #26566 > Yes ExcelFile has it's own engine parameter.

so perhaps ExcelFile should be monkeypatched as well, so that the engine parameterisation applies to all tests?

 there is basically two changes here, the working directory and the engine parameterisation, maybe they should be kept seperate.

the autouse fixture for the working directory is fine.

maybe have the engine parameterisation as a seperate fixture, not make it autouse. then 

param_engine  = pytest.mark.usefixtures('\<fixturename\>') near the top of the module

and then decorate using @param_engine on just the tests to be parametrised ( excluding test_bad_engine_raises etc) A module level fixture would be great, though it's just going to take quite a few more PRs to get there, especially since a lot of the Writer test cases are strangely intertwined with reading. Left yet to go is:

  - Clean up unnecessary WriterBase subclassing (already done in a separate PR)
  - Replace instance variables with fixtures
  - Move tests that require both reading and writing out of WriterBase and into TestRoundTrip
  - Eliminate SharedItems altogether
  - Break tests into a subdirectory

Amongst potentially other things. What you described makes sense but it's going to be rather difficult to move that to the module level while the WriterBase is still subclassing SharedItems as things are convoluted and heavily intertwined where they don't need to be right now :-( 

The scope it's at now reflects current state so trying to minimize movement > A module level fixture would be great,

i wasn't suggesting moving the fixture.

param_engine decorator defined at module level for clarity. this can be defined before the fixture since it's a pytest mark only. the fixture name is a string. but yes it could be defined at class level.

in the past where i have only a few exceptions to a autouse fixture, i created another fixture to undo the monkeypatch and apply to the exceptions. that's a bit more complicated though.

don't we now, as the PR stands, have parameterisation applied to tests that is ignored? and wasn't before and test output descriptions implies parametrisation. > Left yet to go is

i think the tests that pass that shouldn't are different from master. so there is a change in test behavior here. i'll go back and thoroughly double check this.

that's my concern. otherwise the changes are great. > param_engine decorator defined at module level for clarity

OK I think I follow. In the end we probably want separate objects for `read_engine` and `write_engine` which could be used for the various classes, and sure ultimately the combination of them applied in the `TestRoundTrip` class that was introduced. 

> don't we now, as the PR stands, have parameterisation applied to tests that is ignored?

I might not completely follow but generally I'd say no. The majority of parametrization here is for the engine, which most of the tests are using. The exceptions are tests in the WriterBase class that don't need to read in a file, but it's a mixed bag there; they don't belong in that class in any case so if we can decouple those from pure writing tests we can clean this up further, so I think that's one of the follow ups.

Let me know if I misunderstood anything though; this module definitely warrants close review! > i think the tests that pass that shouldn't are different from master. so there is a change in test behavior here. i'll go back and thoroughly double check this.

Let me know what you see. I was getting this on master and this branch:

```sh
======== 966 passed, 12 skipped, 3 xfailed, 2 warnings in 30.45 seconds ========
```
 if i change the fixture

`func = partial(pd.read_excel, engine=request.param)`

to

`func = partial(pd.read_excel, engine='foo')`

i get..

```
$ pytest pandas/tests/io/test_excel.py::TestXlrdReader --tb=no
============================= test session starts =============================
platform win32 -- Python 3.7.3, pytest-4.5.0, py-1.7.0, pluggy-0.11.0
hypothesis profile 'ci' -> timeout=unlimited, deadline=timedelta(milliseconds=500.0), suppress_health_check=[HealthCheck.too_slow], database=DirectoryBasedExampleDatabase('C:\\Users\\simon\\OneDrive\\code\\pandas-simonjayhawkins\\.hypothesis\\examples')
rootdir: C:\Users\simon\OneDrive\code\pandas-simonjayhawkins, inifile: setup.cfg
plugins: xdist-1.28.0, mock-1.10.4, forked-0.2, cov-2.6.1, hypothesis-4.17.2
collected 360 items

pandas\tests\io\test_excel.py FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF [ 11%]
FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF [ 31%]
FFFFFFFFFFFFFFFFFF..............................FFFFFFFFFFFFFFFFFFFFFFFF [ 51%]
FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF......FFFFFF [ 71%]
ssssssFFFFFFFFFFFFFFFFFF......FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF [ 91%]
FFFFFFFFFFFFFFFFFFFFFFFF......                                           [100%]

============================== warnings summary ===============================
pandas/tests/io/test_excel.py::TestXlrdReader::test_usecols_int[xlrd-.xls]
pandas/tests/io/test_excel.py::TestXlrdReader::test_usecols_int[xlrd-.xls]
  C:\Users\simon\Anaconda3\envs\pandas-dev\lib\site-packages\botocore\vendored\requests\packages\urllib3\_collections.py:1: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working
    from collections import Mapping, MutableMapping

-- Docs: https://docs.pytest.org/en/latest/warnings.html
======== 306 failed, 48 passed, 6 skipped, 2 warnings in 27.33 seconds ========
```

48 tests (only 8 actual test) passing. shouldn't they fail if the parametrisation is being applied? > > i think the tests that pass that shouldn't are different from master. so there is a change in test behavior here. i'll go back and thoroughly double check this.
> 
> Let me know what you see. I was getting this on master and this branch:
> 
> ```shell
> ======== 966 passed, 12 skipped, 3 xfailed, 2 warnings in 30.45 seconds ========
> ```

i should have said the tests that pass, after intentionally breaking the fixture. > 48 tests (only 8 actual test) passing. shouldn't they fail if the parametrisation is being applied?

Ha I might have this backwards but note that that particular parametrization only applies to the reading tests. There are other classes like _WriterBase and even top level tests that won't be affected by changing the partial on master..

changing

`new_func = partial(old_func, engine=request.param)`

to 

`new_func = partial(old_func, engine='foo')`

gives

```
$ pytest pandas/tests/io/test_excel.py::TestXlrdReader --tb=no
============================= test session starts =============================
platform win32 -- Python 3.7.3, pytest-4.5.0, py-1.7.0, pluggy-0.11.0
hypothesis profile 'ci' -> timeout=unlimited, deadline=timedelta(milliseconds=500.0), suppress_health_check=[HealthCheck.too_slow], database=DirectoryBasedExampleDatabase('C:\\Users\\simon\\OneDrive\\code\\pandas-simonjayhawkins\\.hypothesis\\examples')
rootdir: C:\Users\simon\OneDrive\code\pandas-simonjayhawkins, inifile: setup.cfg
plugins: xdist-1.28.0, mock-1.10.4, forked-0.2, cov-2.6.1, hypothesis-4.17.2
collected 360 items

pandas\tests\io\test_excel.py FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF [ 11%]
FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF [ 31%]
FFFFFFFFFFFFFFFFFF..............................FFFFFFFFFFFFFFFFFFFFFFFF [ 51%]
FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF............FFFFFF [ 71%]
ssssss........................FFFFFF.................................... [ 91%]
..............................                                           [100%]

============================== warnings summary ===============================
pandas/tests/io/test_excel.py::TestXlrdReader::test_usecols_int[xlrd-.xls]
pandas/tests/io/test_excel.py::TestXlrdReader::test_usecols_int[xlrd-.xls]
  C:\Users\simon\Anaconda3\envs\pandas-dev\lib\site-packages\botocore\vendored\requests\packages\urllib3\_collections.py:1: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working
    from collections import Mapping, MutableMapping

-- Docs: https://docs.pytest.org/en/latest/warnings.html
======= 222 failed, 132 passed, 6 skipped, 2 warnings in 35.83 seconds ========
```

so the test behavior is different. Great catch! But if anything I think it's indicative of an issue on master, where `read_excel` and `pd.read_excel` are used rather interchangeably but with different results. 

Can you run verbose output and see what the difference in passing in? My guess is that it's function in the ReadingTestsBase which previously used `pd.read_excel` and may inadvertently not have gotten parametrized the tests that are not observing the parametrisation in this PR are

```
test_excel_passes_na

test_unexpected_kwargs_raises

test_excel_table_sheet_by_index

test_bad_engine_raises

test_reader_closes_file

test_read_xlrd_book
```
these tests are also not failing (after intentianally breaking parameterisation) on master, so i've satisfied myself that the problem is not a new one.  test_bad_engine_raises shouldn't parametrised.

the other tests all use ExcelFile.

there are some other tests that use ExcelFile that aren't appearing in the list because they are tests that will need to be split (future PR). they have a read_excel preceeding the ExcelFile and not showing with this testing of the tests.

I'm happy to approve this PR on two grounds:

firstly the problems are not introduced by this PR and secondly we are only testing one engine.

before i do, do you want to see if you can easily monkeypatch ExcelFile? I don’t quite understand what you are asking for with the monkeypatch of ExcelFile - is that trying to solve a problem or expand coverage?

Sent from my iPhone

> On May 29, 2019, at 3:07 PM, Simon Hawkins <notifications@github.com> wrote:
> 
> test_bad_engine_raises shouldn't parametrised.
> 
> the other tests all use ExcelFile.
> 
> there are some other tests that use ExcelFile that aren't appearing in the list because they are tests that will need to be split (future PR). they have a read_excel preceeding the ExcelFile and not showing with this testing of the tests.
> 
> I'm happy to approve this PR on two grounds:
> 
> firstly the problems are not introduced by this PR and secondly we are only testing one engine.
> 
> before i do, do you want to see if you can easily monkeypatch ExcelFile?
> 
> —
> You are receiving this because you were mentioned.
> Reply to this email directly, view it on GitHub, or mute the thread.
 i give it a go in a short while, can partial be used on the Class constructor? I believe partial is only for functions not classes. 

Is your question to convert `get_csv_refdf` into a fixture? > Is your question to convert `get_csv_refdf` into a fixture?

i was thinking we could add another monkeypatch.setattr to the current cd_and_set_engine fixture

> I believe partial is only for functions not classes.

not on the __init__ method then?
 > Great catch! But if anything I think it's indicative of an issue on master, where `read_excel` and `pd.read_excel` are used rather interchangeably but with different results.
> 
> Can you run verbose output and see what the difference in passing in? My guess is that it's function in the ReadingTestsBase which previously used `pd.read_excel` and may inadvertently not have gotten parametrized

on master the following tests are all using read_excel directly and not self.get_exceldf where the parameterisation was being applied. hence not being parametrised as intended.

```
test_excel_read_buffer
test_read_from_file_url
test_read_from_pathlib_path
test_read_from_py_localpath
test_read_excel_multiindex
test_read_excel_multiindex_header_only
test_excel_old_index_format
test_read_excel_bool_header_arg
test_read_excel_chunksize
test_read_excel_skiprows_list
test_read_excel_nrows
test_read_excel_nrows_greater_than_nrows_in_file
test_read_excel_nrows_non_integer_parameter
test_read_excel_squeeze
```

So this PR has fixed those issues.
 ```
        new_init = partialmethod(ExcelFile.__init__, engine='foo')
        monkeypatch.setattr(ExcelFile, '__init__', new_init)
```
 is working on all the tests apart from test_read_xlrd_book (and test_bad_engine_raises), i'll keep digging the non-foo version!

```
        new_init = partialmethod(ExcelFile.__init__, engine=request.param)
        monkeypatch.setattr(ExcelFile, '__init__', new_init)
``` > is working on all the tests apart from test_read_xlrd_book

so this test is included in the parametrisation of the class, but specifies the engine explicitly as engine = "xlrd"

removing that works. The only reader right now is xlrd. If the monkey patching you are looking at is only for the readers I think it should wait until another PR or until we actually get another reading engine implemented.  lgtm. merge when ready @simonjayhawkins  and @WillAyd 