How can I get source code of imported libraries functions?

Good time of day!

I am developing a plugin for PyCharm that analyzes functions code and predicts inputs and outputs for it. For now I am only available to get methods that were written inside a project (by developer). I do it in a following way [I also suggest "suitable" variables] (I will not attach all the code, because there are lots of not needed stuff):

@Override
protected void addCompletions(@NotNull CompletionParameters parameters,
@NotNull ProcessingContext context,
@NotNull CompletionResultSet result) {
.....
CaretModel caret = parameters.getEditor().getCaretModel();
PsiElement currentElem = parameters.getOriginalFile().findElementAt(caret.getOffset());
....
PyCallExpression callExpression = PsiTreeUtil.getParentOfType(currentElem, PyCallExpression.class);
PyExpression callee = Objects.requireNonNull(callExpression).getCallee();
.....
PyGotoDeclarationHandler handler = new PyGotoDeclarationHandler();
PsiElement pyFunction = handler.getGotoDeclarationTarget(callee, parameters.getEditor());
.....
String functionCode = pyFunction.getText()

The question is: how can I access functions that are written in libraries?
Let's say, that user somewhere inside code imported tensorflow library in a next way:

import tensorflow as tf

and later he used:

tf.exp(5)

My aim is to get the code of the tf.exp() function that is:

def exp(self, context=None):
    """Returns e ** self."""

    if context is None:
        context = getcontext()

    # exp(NaN) = NaN
    ans = self._check_nans(context=context)
    if ans:
        return ans
....
return ans

I tried playing with scopes and FindUsagesProvider but didn't succeed. Another my try was connected to Documentation providing, but for such case I need an original PsiElement which I don't know ho to get. Also, I thought about using Psi Files, but didn't find a way out.

How can I get all "def " functions? Not obligatory with PyGotoDeclarationHandler. I hope that there exists some universal solution :)

Thank's in advance!