pyAT 'Multipole has no attribute PolynomB' exception for loading multipoles from a .mat file with both PolynomB and K value defined

[space-budgie]

Issue

While trying to load a .mat lattice file using at.load.matfile.load_mat, multipole magnets that have both a PolynomB and a K value defined raise an error:

AttributeError: In element QFEND, parameter K: 'Multipole' object has no attribute 'PolynomB'

This was tested for Python accelerator toolbox version 0.4.0.

Here, QFEND is defined with with the Matlab code

QFEND 	= atmultipole('QFEND' , 0.250000, [0 0 0 0], [0  3.653849 0 0], 'StrMPoleSymplectic4Pass','MaxOrder',2);  
QFEND.K   = QFEND.PolynomB(2);

Potential cause

I believe that the issue lies in the function call

at/pyat/at/lattice/elements.py

Line 628 in 1e32037

super(ThinMultipole, self).__init__(family_name, **kwargs)

where passing in K as part of **kwargs causes a call to

at/pyat/at/lattice/elements.py

Lines 704 to 706 in 1e32037

	@K.setter
	def K(self, strength: float):
	self.PolynomB[1] = strength

but PolynomB is only defined later in

at/pyat/at/lattice/elements.py

Line 634 in 1e32037

self.PolynomB = lengthen(poly_b, len_ab - len_b)

(Temporary) fix

I managed to bypass the issue by changing the lines

at/pyat/at/lattice/elements.py

Lines 628 to 634 in 1e32037

	super(ThinMultipole, self).__init__(family_name, **kwargs)
	# Set MaxOrder while PolynomA and PolynomB are not set yet
	super(ThinMultipole, self).__setattr__('MaxOrder', maxorder)
	# Adjust polynom lengths and set them
	len_ab = max(self.MaxOrder + 1, len_a, len_b)
	self.PolynomA = lengthen(poly_a, len_ab - len_a)
	self.PolynomB = lengthen(poly_b, len_ab - len_b)

to

        # Set MaxOrder while PolynomA and PolynomB are not set yet
        super(ThinMultipole, self).__setattr__('MaxOrder', maxorder)
        # Adjust polynom lengths and set them
        len_ab = max(self.MaxOrder + 1, len_a, len_b)
        self.PolynomA = lengthen(poly_a, len_ab - len_a)
        self.PolynomB = lengthen(poly_b, len_ab - len_b)
        super(ThinMultipole, self).__init__(family_name, **kwargs)

so that PolynomB is defined prior to the potential call to the setter.

However, I have not tested if this breaks anything else in the code.

[T-Nicholls]

Hello @space-budgie, thank you for finding this bug and clearly identifying the cause.

Interestingly, in the example you've given, if atquadrupole had been used to create the element in Matlab instead of atmultipole then this bug wouldn't have occurred; I'm glad you didn't though as this exposes a deeper issue with how we handle K and H attributes/kwargs during element creation.

Good job with the temporary fix and for being aware that it may break other things; while, strictly speaking, it doesn't break anything else, it would result in the value of K overwriting PolynomB[1] which is the opposite of how it is currently handled and would be inconsistent with the other magnet classes.

I will prepare a complete fix that correctly handles K and H attributes during element creation for all applicable magnet types and make a pull request.