Refactor is_linear_expression function of expr.py

[alisa-kirkinskaia]

1. Fix #121:

Fixed a bug in is_linear_expression related to expand() being very slow for some expressions. The bug is caused by the following line:

if not( (a-b).expand() == 0 or a.expand() == b.expand()):

In the fix we avoid comparing the values of a.expand() and b.expand() if (a-b).expand() is 0, as it already means the expression is linear.

2. Refactor is_linear_expression for better readability:

• Remove unused argument integral
• Rename variables
• Add docstring

3. Update library patch version to 0.19.3

[yguclu]

Can you please update the library version to 0.19.3 in the file sympde/pyproject.toml?

[yguclu]

Good job @alisa-kirkinskaia! I have a couple of minor comments, mostly about formatting.

[yguclu]

I think expr should be of some basic SymPy type. Could you please try and find out which one? It would be nice to add this information to the docstring, and possibly do a type check, too.

[yguclu]

We should check that is_linear_expression truly works for expressions with an arbitrary number of arguments. I am not sure if we do that already in any unit tests in folder sympde/expr/, could you please have a look?

[yguclu]

Minor formatting improvements:

Suggested change

	expr_at_sum = expr.subs(zip(args, summed_args)) # f(x + y)
	expected_sum = expr_at_x + expr_at_y # = f(x) + f (y)
	expr_at_sum = expr.subs(zip(args, summed_args)) # f(x + y)
	expected_sum = expr_at_x + expr_at_y # f(x) + f(y)

[yguclu]

For long expressions, this error message will be quite difficult to read. Hence, I suggest expanding it across multiple lines:

Suggested change

	print('{} != {}'.format(expr1, expr2))
	print(r"Failed to assert addition property `f(x + y) = f(x) + f(y)`, where:")
	print()
	print('f(x + y) =')
	print(expr1)
	print()
	print('f(x) + f(y) =')
	print(expr2)

Please note that I have used a Python raw string in the first line of the suggested code, in order to avoid any issue with the markdown inline quote characters I wanted to print

[yguclu]

Here I also suggest splitting the error message across multiple lines, as in my previous commentLOL.

[yguclu]

It is probably worth explaining in some detail that x and y at the top of this docstring correspond to different values of args, which is the list of arguments expr depends on

[yguclu]

Can you please update the library version to 0.19.3 in the file sympde/pyproject.toml?

Thanks for commit 85fba96!