Fix errors='ignore' being ignored in astype #30324
- [x] closes #30324 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Hello @naomi172839! 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 2020-03-05 03:46:10 UTC @WillAyd I added unit tests to test_generic.py.  I have never used pytest before and am having trouble building the c components to test it myself.  Hopefully it was done right. I seemed to have messed up the GIT... Let me know if I need to start over:/ > I seemed to have messed up the GIT... Let me know if I need to start over:/

You can still use this PR/branch - fix it up locally and use —force if needed OK so I fixed the git.  The tests actually seemed to already exist to make sure that the error was thrown.  I am not sure why it did not fail before.   can you add a whatsnew note @jreback When I add my own whatsnew entry it seems to create a conflict that I am unsure how to resolve.  I already updated my local whatsnew file to the latest version. Hmm, looking back at the original issue, I'm not sure if this is what we want to do.

The `errors` keyword is used to handle raising when there's in issue with the conversion of values. This PR overloads `errors` to handle raising when there are non-conversion issues (incorrect keys in `dtype` dict for example). I don't think we want to overload the meaning of `errors` like that. Apologies for not catching that earlier @naomi172839. @tomaugspurger would you then suggest adding an additional parameter to the astype method to allow the user to ignore the KeyErrors? That's one option, though I'm not sure I'd find it useful enough. Others may though. Perhaps @climatebrad can share his original usecase. My [usecase](https://github.com/climatebrad/stop-and-frisk/) is that I was joining several dataframes generated from different years of NYPD stop-and-frisk datasets into one multiyear dataframe. A lot of the work to keep the dataframes manageable was making sure that categorical data was saved as such. So I had a list of all of the columns that needed to be categorical data and was setting them. (Technically I converted to object type, did more processing, then converted to category type.) This code failed:

```
cat_set = set(CLEAN_CAT_VALUES) | set(CAT_FILL_NA_VALUES) 
data = data.astype({cat : 'object' for cat in cat_set}, errors='ignore')
```

Here's the diff where I dealt with what I consider the incorrect `errors='ignore'` behavior. (I had to explicitly exclude columns that didn't exist in the dataframe I was editing.) Not the hardest workaround, but I do still think that `errors='ignore'` should, you know, ignore errors.

https://github.com/climatebrad/stop-and-frisk/commit/d755084c100fd0f8f8d883f9f9a471ddbb0a5c02#diff-a0ebe5d9008a8b884ea03629345f9735R178

```
cat_set = set(CLEAN_CAT_VALUES) | set(CAT_FILL_NA_VALUES) 
cat_set = cat_set.intersection(set(data.columns))
data = data.astype({cat : 'object' for cat in cat_set}, errors='ignore')
``` @climatebrad I do agree that errors='ignore' at least implies that errors will be ignored but @tomaugspurger does have a point.  It would overload the method and that's probably not the best solution. Looks like the 1.0 whatsnew got mangled in this - can you check that out from master? Sorry that it took so long, I had to figure out how to check out a single file. @naomi172839 not sure why the whatsnew files are collected here, but can you try the following on your branch?

```python
git fetch upstream/master
git checkout upstream/master pandas/doc/source/whatsnew/v1*
```

Then re-push? I think should fix the issues here (can add a whatsnew note after we fix these up) @WillAyd So I tried that with no real luck.  I tried deleting the files and rechecking them out which got me to where we are now.  I dont have high hopes. Your last push looks a lot better. The only outstanding issue is that the file permissions have changed for the v1.0.0 whatsnew.

If you do `git checkout upstream/master pandas/doc/source/whatsnew/v1.0.0.rst` now does `git status` show anything as staged? Output is below:
nbonnin@Naomis-MacBook-Pro pandas % git checkout upstream/master doc/source/whatsnew/v1.0.0.rst
Updated 0 paths from 247718065
nbonnin@Naomis-MacBook-Pro pandas % git status
On branch #30324
Your branch is up to date with 'origin/#30324'.

nothing to commit, working tree clean

So I don't think that there is anything staged Hmm OK. I guess git checkout might not grab file permissions.

Since you are on macOS you can just do `chmod 1644 pandas/doc/source/whatsnew/v1.0.0.rst` instead to restore back to where it was Done, though its not really showing any changes and zero differences from upstream/master for that file Is your `upstream` pointing to pandas? If not sure, can you post the output of `git remote -v show`? Output 
nbonnin@Naomis-MacBook-Pro pandas % git remote -v show
origin  https://github.com/naomi172839/pandas.git (fetch)
origin  https://github.com/naomi172839/pandas.git (push)
upstream        https://github.com/pandas-dev/pandas.git (fetch)
upstream        https://github.com/pandas-dev/pandas.git (push)
 Just for posterity try this one more time - it worked for the other whatsnew files so not sure what makes the 1.0.0 any different but maybe the changed permissions prevented this from working

```sh
git fetch upstream/master
git checkout upstream/master pandas/doc/source/whatsnew/v1.0.0.rst
``` Doing that, git thought that there were changes so I went ahead and pushed it. Hmm still didn't work. If you run `git diff upstream/master --name-only` locally none of the whatsnew files should show, but they seem to here on GitHub

If all else fails maybe just download the file directly from GitHub Output:
...
doc/source/whatsnew/v0.21.0.rst
doc/source/whatsnew/v0.23.0.rst
doc/source/whatsnew/v0.25.0.rst
doc/source/whatsnew/v0.25.3.rst
doc/source/whatsnew/v1.0.1.rst
doc/source/whatsnew/v1.0.2.rst
doc/source/whatsnew/v1.1.0.rst
...

v1.0.0.rst isn't listed.  Nevertheless I tried downloading straight from github.  It still seems to have made no difference.
 Hmm OK. Actually just needed to do a `git merge upstream/master` and repush

I did that to fix things up here, so just `git pull` from your local branch to grab the changes and should be able to move forward Ok great!  that did alot.  At this point it should be just adding the what's new message correct?  I'm not super great at git (obviously).  Just want to make sure before I push another commit.  Hmm just reading back through the comment history I do agree with @TomAugspurger that this blurs the purpose of the `errors` keyword; any chance you thought about alternative implementations?