Remove std::endl from print functions

[p-zach]

Issue

Currently, classes append std::endl in their print statements, such as Pose2:

void Pose2::print(const string& s) const {
  std::cout << (s.empty() ? s : s + " ") << *this << std::endl;
}

It seems to me that this is unnecessary and goes against best practice. It means that users have less control over how they can format their print statements. For example, I would have liked the Python code

print(f"{global_point} transformed by {origin} into local space -> {transformed}")` 

to yield

[5. 5.] transformed by (1, 1, 3.14159) into local space -> [-4. -4.]

but instead it gives the following:

This example also demonstrates best practice as followed by Numpy, which does not automatically insert newlines.

Solution

Remove std::endl from print functions. Perhaps it can be implemented so that the endline is only added if the user has not supplied a prefix string. I can create a PR if this solution is desired.

Caveats

This is an opinionated issue, but I think that allowing the user to format their strings how they wish is strictly better than the current approach. The optional string prefix is handy for debugging and writing quick statements, but for even slightly more elaborate formatting it gets in the way.

[dellaert]

I sympathize, but I don't want to break existing workflows on current behavior. However, there is a solution: phase out print (in a few releases) in favor of streaming. Some classes already support ostream, with no prefix or endl. If you wanted to make a PR with that (and where you can use it in print, to avoid duplication), that would be a very worthwhile contribution. Could be done one subfolder at a time, e.g, starting with geometry.

[dellaert]

PS how to surface that in python to support {} would have to be looked into; the wrapper should maybe hook up to streaming rather than print.

[p-zach]

I see what you mean. For future reference, I think the place to modify would be

gtsam/wrap/gtwrap/pybind_wrapper.py

Line 142 in 814a734

ret += '''{prefix}.def("__repr__",

[varunagrawal]

PS how to surface that in python to support {} would have to be looked into; the wrapper should maybe hook up to streaming rather than print.

This is easy to do in the wrapper once we have full streaming support. :)

[ProfFan]

The current python printing is a shortcut to just wrap the old print() functions. One probable next step is moving all print methods to proper formatters with fmt (I have prototyped this a few years ago) and rewrite the existing print() functions to just call the fmt formatter.

[ProfFan]

Furthermore, we can (and should!) support the custom formatting in Python with "{:.2f}".format(pose3) style by overloading the __format__(self, format_str) method.

BTW the fmt::format(fmtlib) or std::format (C++23) style of logging/printing has been everywhere now. But ostream is still much much better than print.