How to copy all fields's value from one record into another?

I have a existing record A and I want cover its value from record B. In regular way, we use A.write({'filed1':RecB's_value,...}) to update. But the number of fields near 400, I was wonder that if there is a convenient way to do that.

I have tried that as code below

copy = rec_b.copy()
rec_a.write({copy})

got error

AttributeError: 'yc.purchase' object has no attribute 'pop'

I expected that can instead my manual assignment.

I found solution, hope it can help who have same issue.

a = self.browse(a)
b = self.browse(b)
# read all field_name into a list
_fields = []
for fn in self._proper_fields._map.keys():
    _fields.append(fn)
# start to write
vals = {}
for _f in _fields:
    # M2O must use id
    if hasattr(b[_f],'id'):
        # a.write({_f: b[_f].id})
        vals.update({_f: b[_f].id})
    else:
        # a.write({_f: b[_f]})
        vals.update({_f: b[_f]})

a.write(vals)