[Bug]: Caching bugs when accessing context in Sentence

[alanakbik]

Describe the bug

The right_context and left_context functions of the Sentence are used to calculate the context for all our best models. However, the behavior of the context expansion is inconsistent due to two bugs.

To Reproduce

from flair.data import Sentence

# make a sentence without context
sentence = Sentence("Luke and Leia destroyed the Death Star.")
print(sentence)
# print right context: There is none - CORRECT!
print(sentence.right_context(4))

# now make a second sentence and set it as context
other_sentence = Sentence("The Death Star then exploded.")
Sentence.set_context_for_sentences([sentence, other_sentence])
# print(sentence.next_sentence()) # verify that next sentence is correctly set (it is: CORRECT)

# now print right context. Even though context is set, right_context returns '[]': ERROR!
print(sentence.right_context(4))

# now calculate right context for some other random sentence
Sentence("Why am I here?").right_context(4)

# print right context again. Now suddenly, the correct context is returned. (ERROR because inconsistent behavior)
print(sentence.right_context(4))

Expected behaivor

The correct context should always be returned.

Additional Context

There are likely two reasons for this:

• We set the lru_cache of right_context to 1 (

  flair/flair/data.py

  Line 830 in 857337d

  @lru_cache(maxsize=1) # cache last context, as training repeats calls

  ). This is because I assumed that caching is computed here per-instance. But it turns out that caching is computed globally, even though the method is part of Sentence. This is why computing the right_context for some random sentence as in the snippet above results in a different context being computed: The original cache is already lost. -> to fix, set much higher cache size here
• The main error is likely a problem with the equality definition of Sentence. Equality is considered only using features of the sentence itself, not its context. This is why setting a context belately and then calling right_context again gives the same result as before -> to fix this, the quality definition of Sentence needs to be changed

Environment

Python 3.8, master branch

[alanakbik]

Closed by #3157