@farah-salama the print statements existed for checking the tests. Please revert that change.

@Shaikh-Ubaid @kmr-srbh Thank you so much for your help.
I tried to handle the compile-time implementation for the functions in handle_constant_string_attributes but I couldn't find a similar function implemented that takes an integer argument and I faced many errors that I couldn't solve so I changed the test cases to test the run time implementation only, I would appreciate it though if someone could tell me how to handle the compile-time implementation for such functions.

@farah-salama I present the method to implement center. You may similarly follow to implement expandtabs.

We first check for the number of arguments to the function.

if (args.size() != 1 || args.size() != 2) {
    throw SemanticError("str.center() takes 1 or 2 arguments",
                        loc);
}

We next move on to check the argument types. As fillchar is an optional parameter, we need an if block to check for args.size() again. When we have 1 argument, we handle only width. When we have 2 arguments, we handle both.

You can check for width being an integer type like:

ASR::expr_t *arg1 = args[0].m_value;
ASR::ttype_t *type = ASRUtils::expr_type(arg1);
if (!ASRUtils::is_integer(*type)) {
    throw SemanticError("argument 'width' to str.center() must be of type int",
                    arg1->base.loc);
}

We follow the above step again to check for args[1].m_value when we have 2 arguments.

We next check whether the provided arguments are compile-time constants. Handle this according to the arguments you are handling above. We do so by checking if they are not all nullptr:

if (ASRUtils::expr_value(arg1) != nullptr) {...}

We next store their compile time values. We do this here because we need all the arguments to be a compile-time constant to move ahead. For the width parameter of center, we can do:

ASR::IntegerConstant_t* width_arg = ASR::down_cast<ASR::IntegerConstant_t>(arg1);
int64_t width = width_arg->m_n;

fillchar is an optional parameter so we declare it as std::string fillchar = " "; when only 1 argument is provided. We follow the above step to store it as a StringConstant_t* when we have 2 arguments.

Now you need to implement the same logic you have used for implementing the method.
We need an else clause to handle cases when we have either one or all arguments non-constants. Inside that clause, we make a call to the _lpython_str_center function which you have already implemented. You may have a look at how endswith does this.

Finally, make a new StringConstant_t with the obtained value and set it to tmp.

This is a rough sketch, please modify it accordingly.

@farah-salama Please mark "Ready for review" when ready.