Question: I have a pandas DataFrame `df` that looks like this:

```
A      B
0  ['a','b']
1  ['c','d']
2  ['a','c']
3  ['b','d']
4  ['a','d']

```

Now, I wish to subset `df` by only selecting those rows in which `'a'` belongs to the list in `B`, the desired output being:

```
A      B
0  ['a','b']
2  ['a','c'] 
4  ['a','d']

```

Naively, I tried `df['a' in df['B']]`, but this doesn't seem to work. How would I go about doing this?

Answer: 
After debugging the issue, using <https://developers.pinterest.com/tools/url-debugger/>, the problem seems to stem from the server-side. 

> 
> If you're seeing We weren't able to retrieve any data from your URL,
>  then Pinterest may be timing out before we can create a Pin on your
>  site. Unfortunately, this isn't something we can fix on our end, and
>  usually happens when a site is slow. This error also appears if your
>  site blocks Amazon Web Services (AWS) requests. You'll have to remove
>  this block before applying for Rich Pins.
> 
> 
> 

Creating some "minimal but working" pages and running them through the validator made the issue clearer, but unfortunately didn't resolve it. Either this is a problem with the local server (Apache) or possibly an issue with the pinterest bot.

The Pinterest bot seems to hit the server OK, but the validator can't parse or reach the endpoint. Since the minimal examples returns in ~less than a second, and contains a bare minimum to satisfy Pinterests requirements, it's clear that the issue is not within your code. 

Migrating the solution to another host seems to have fixed the problem. 

**Edit**

The new host seems to be working better (using one of the minimal examples):
[![enter image description here](https://i.stack.imgur.com/XhsyM.png)](https://i.stack.imgur.com/XhsyM.png)
For me the problem was I was using "<http://example.com>". The validator wouldn't recognize my site. Not AT ALL. Not even the site name or the favicon. Then I switched to "<http://www.example.com>" and it worked perfectly.