Setting manual tick labels does not set manual ticks. It just labels the first n ticks where n is your number of tick labels. We suggest using set_ticks with the label argument to prevent confusion

Setting manual tick labels does not set manual ticks. It just labels the first n ticks where n is your number of tick labels. We suggest using set_ticks with the label argument to prevent confusion

Yes, that part is clear. In reality set_ticklabels is called only to format the original labels at their original location. I just needed a minimal example that reproduces the original bug. The set ticks part is omitted deliberately in this example code. Otherwise the bug disappears.

If you just want to format the labels for automatically placed ticks, you might want to set a tick formatter, rather than explicitly setting labels:
https://matplotlib.org/stable/gallery/ticks/tick-formatters.html

import numpy as np
from matplotlib import cm
import matplotlib.pyplot as plt

for adjust in [False, True]:
    fig, ax = plt.subplots(figsize=(5, 3))
    data = [33.4, 33.5, 33.6]
    ax.plot(data, data)
    ticks = [32.8, 33., 33.2, 33.4, 33.6, 33.8]
    # ax.set_yticks(ticks)
    # ax.set_yticklabels(map(str, ticks))

    if adjust:
        fig.subplots_adjust(top=0.96)
        fig.savefig('Adjust.png')
    else:
        fig.savefig('DontAdjust.png')

If you adjust, you get twice as many ticks:

Don't Adjust

Adjust

You then label them with the un-adjuested (too few) labels:

You further get a Warning

UserWarning: FixedFormatter should only be used together with FixedLocator
  ax.set_yticklabels(map(str, ticks))

I think the warning could be less mysterious. Note that https://matplotlib.org/stable/api/_as_gen/matplotlib.axes.Axes.set_xticklabels.html is discouraged in the docs.

To summarize:

• set_xticklabels switches to using FixedFormammter which maps your N strings to the first n ticks. In some limited cases this is useful (e.g. a stand alone script where you can trust the number of ticks won't change because the next thing you do is save without a layout manager)
• adjusting the axes size in the OP's example happens to fall across an edge where the auto locator decides to increase the number of ticks. Changing the figure size or running one of the layout engines has a good chance of generating the same issue (or reducing the number of ticks and making essentially the same error but in a less detectable way).

So I think our options are:

1. do nothing, we are warning and this is, although a foot-gun, expected behavior
2. make the warning clearer / special case it in set_ticklabels to give more context as to how to fix it
3. just...fix it? We can detect when we are setting a FixedFormatter with not-a-FixedLocator...can we just grab the current tick locations, set a FixedLocator, and warn loudly?
4. deprecate set_ticklabels because it is a persistent foot-cannon
5. make it raise instead of warn

Of the options, I'm leaning towards 3. If we do just "fix it", I suspect that it can be done in a way that has 0 effect on people who were getting lucky and protect users from falling into this trap. 1 is not great (it is biting users), 2 is like the status quo but just more helpfully broken, 4 and 5 will break a bunch of users for whom things are currently "working" (and these may be low-Matplotlib-expertise users as they are using something we advise against and are currently ignoring a warning).

I don't understand how you will "just fix it"? That's what we do now I think: There are six ticks and the user specifies four labels so two end up blank.

In special cases we could infer the tick values from the strings, but that's not true in general.

I started a PR to just have a specific warning in set_ticklabels that doesn't refer to mysterious classes like FixedFormatter.

In the case where there are N ticks and the user gives us N labels, we can change what ever locator is there into a FixedLocator with where the ticks currently are

I believe that is what we already do. The issue here is that the number of ticks changes after the user has set the tick labels when the Axes is made larger.

We set a FixedFormammter, if we had also set a FixedLocator then the tick locations should not have changed

You mean if the set_ticks happens before the resize? That could work if the order of calls is correct. I think we'd have to trigger a draw to get the ticks from the AutoLocator at that point?