Using swig Python wrapper: argument 2 of type 'std::unordered_set< std::string > const &'

Question

I am updating an old project using (parent project of) the Python package coqui_stt_ctcdecoder generated from C++ using SWIG. Some parameter types in some methods have changed. I am stuck with the method Scorer::fill_dictionary that takes const std::unordered_set<std::string>& as an argument in C++. In the old Python code a list of bytes is passed, but that no longer works, as well as a set. I have no idea what type to put there. The error is

Traceback (most recent call last):
  File "/mnt/d/shared/speech/dsalign/STT-align/align/align.py", line 693, in <module>
    main()
  File "/mnt/d/shared/speech/dsalign/STT-align/align/align.py", line 451, in main
    create_bundle(alphabet_path, scorer_path + '.' + 'lm.binary', scorer_path + '.' + 'vocab-500000.txt', scorer_path, False, 0.931289039105002, 1.1834137581510284)
  File "/mnt/d/shared/speech/dsalign/STT-align/align/generate_package.py", line 75, in create_bundle
    scorer.fill_dictionary(words)
  File "/mnt/d/shared/speech/dsalign/STT-align/venv/lib/python3.10/site-packages/coqui_stt_ctcdecoder/swigwrapper.py", line 1269, in fill_dictionary
    return _swigwrapper.Scorer_fill_dictionary(self, vocabulary)
TypeError: in method 'Scorer_fill_dictionary', argument 2 of type 'std::unordered_set< std::string > const &'

EDIT: I have tried a list and a set of str and bytes, all with the above exception. I have used Python 3.8 on Windows and WSL.

Answer 1

SWIG includes support for std::unordered_set, but note that the interface oddly doesn't accept Python set objects. tuple and list work, however.

Tested example:

test.i

%module test

// Code injected into wrapper
%{
#include <iostream>
#include <string>
#include <unordered_set>

// Function using the parameter type from OP error message
void func(std::unordered_set<std::string> const &string_set) {
    for(auto& s : string_set)
        std::cout << s << std::endl;
}
%}

// SWIG support for templates
%include <std_unordered_set.i>
%include <std_string.i>
// Must instantiate the specific template used
%template(string_set) std::unordered_set<std::string>;

// Tell SWIG to wrap the function
void func(std::unordered_set<std::string> const &string_set);

Demo:

>>> import test
>>> test.func(['abc','def','abc','ghi'])  # list works
abc
def
ghi
>>> s = test.string_set(['aaa','bbb','ccc','aaa','bbb'])
>>> test.func(s)
aaa
bbb
ccc
>>> s = test.string_set(('aaa','bbb','ccc','aaa','bbb')) # tuple works
>>> list(s)
['aaa', 'bbb', 'ccc']
>>> test.func({'abc','def'})  # set doesn't work
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\test.py", line 194, in func
    return _test.func(string_set)
TypeError: in method 'func', argument 1 of type 'std::unordered_set< std::string,std::hash< std::string >,std::equal_to< std::string >,std::allocator< std::string > > const &'