Added repr string for Grouper and TimeGrouper
State: closed

- [ ] closes #xxxx
- [ ] tests added / passed
- [x ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

This is a new implementation of #17727. This is much simpler than the previous attempts. The outputs are now:

```python
>>> pd.Grouper(key='A')
Grouper(key='A')
>>> pd.Grouper(key='A', freq='2T')
TimeGrouper(key='A', freq=<2 * Minutes>, sort=True, closed='left', label='left', convention='e')
```

Tests still need to be written. I can do them when/if this is ok.

Labels:
name: Groupby
name: Output-Formatting
description: __repr__ of pandas objects, to_string
name: Resample
description: resample method

Comments:
Hello @topper-123! Thanks for updating the PR.









Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on November 27, 2017 at 01:12 Hours UTC
After changes according to @jreback, the output is now:

```python
>>> pd.Grouper(key='A')
Grouper(key='A', level=None, freq=None, axis=0, sort=False)
>>> pd.Grouper(key='A', freq='H')
TimeGrouper(key='A', level=None, freq=<Hour>, sort=True, closed='left', label='left', how='mean', nperiods=None, axis=0, fill_method=None, limit=None, loffset=None, kind=None, convention='e', base=0)
```
so what you can do is skip printing attributes that are None
makes it’s shorter w/o loss of generality
can you rebase
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18203?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@38f41e6`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18203/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/18203?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #18203   +/-   ##
=========================================
  Coverage          ?   91.38%           
=========================================
  Files             ?      163           
  Lines             ?    50066           
  Branches          ?        0           
=========================================
  Hits              ?    45753           
  Misses            ?     4313           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.19% <100%> (?)` | |
| #single | `40.36% <100%> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18203?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/18203?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS5weQ==) | `92.03% <100%> (ø)` | |
| [pandas/core/resample.py](https://codecov.io/gh/pandas-dev/pandas/pull/18203?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzYW1wbGUucHk=) | `96.16% <100%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18203?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/18203?src=pr&el=footer). Last update [38f41e6...833883f](https://codecov.io/gh/pandas-dev/pandas/pull/18203?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/18203?src=pr&el=h1) Report
> Merging [#18203](https://codecov.io/gh/pandas-dev/pandas/pull/18203?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f745e52e168790bff06a55928c3491cdea389508?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18203/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/18203?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18203      +/-   ##
==========================================
- Coverage   91.35%   91.33%   -0.02%     
==========================================
  Files         163      163              
  Lines       49796    49797       +1     
==========================================
- Hits        45490    45482       -8     
- Misses       4306     4315       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.13% <100%> (ø)` | :arrow_up: |
| #single | `40.79% <100%> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18203?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/18203?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS5weQ==) | `92.03% <100%> (ø)` | :arrow_up: |
| [pandas/core/resample.py](https://codecov.io/gh/pandas-dev/pandas/pull/18203?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzYW1wbGUucHk=) | `96.34% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18203?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18203?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.81% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18203?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/18203?src=pr&el=footer). Last update [f745e52...bdc5ac2](https://codecov.io/gh/pandas-dev/pandas/pull/18203?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).

green.
linting issue. otherwise lgtm. note that you can do

``make lint-diff`` to see linting in your patch, locally
All green, @jreback 
thanks!