API: ban mutation within groupby.apply

[jreback]

xref #8662
xref #12652

This makes pandas code jump thru hoops and is a complete anti-pattern for pandas. let's nuke it. users should never do this.

[jbrockmendel]

agreed this is a footgun. brainstorming how to implement:

• document that it isnt supported and be done with it
• maybe a contextmanager to make the relevant objects immutable within the relevant calls

[rhshadrach]

@jbrockmendel with the contextmanager, are you thinking doing a comparison after the apply call to see if the objects involved changed? Is there a way to avoid a copy here?

[jbrockmendel]

with the contextmanager, are you thinking doing a comparison after the apply call to see if the objects involved changed?

I am thinking of setting the array.flags.writeable to False for any ndarrays contained inside the DataFrame. We'd need to implement something analogous for ExtensionArrays.