How to find possible subjects for given verb in everyday object domain
If you want something quick, I think pattern is the best tool for the job. It provides a ready-to-use multilingual parser that you can use in the following way:

import pattern
from pattern.en import parse
s = 'I put water in the vase'
s = parse(s)
print s
# output = I/PRP/B-NP/O put/VBP/B-VP/O water/NN/B-NP/O in/IN/B-PP/B-PNP the/DT/B-NP/I-PNP vase/NN/I-NP/I-PNP


Once you have a string like output above, you only need regex parsing to extract every sequence of tokens whose tags match the sequence [B-NP, B-VP, B-NP].

NP stands for "noun phrase" and VP stands for "verb phrase". In English, virtually every sequence consisting of a noun phrase, a verb phrase, and a second noun phrase, all in strict adjacency, is a subject-verb-object sequence, so this should give you what you're looking for.

pattern's parser will also be able to handle some non-strict adjacencies (e.g. intervening adverbs and adjectives between the three phrases in the subject-verb-object sequence).

However, pattern is not terribly sophisticated -this will give you some Precision and some Recall, but not terribly high numbers. If you need high-quality parsing, you should try the Stanford parser's Python implementation or spacy.

Hope this helps!