How to factorize a noncommuting symbol?

[Gerardo Suarez]

So basically I have a lot of objects like H:

a= sympy.Symbol('a', commutative=False)

beta= sympy.Symbol('beta',commutative=True)

gamma= sympy.Symbol('\gamma',commutative=True)

ad = sympy.Symbol('a^{\dagger}', commutative=False)

H=gamma*ad*a+beta*ad*a

and all I wanna do is rewrite it as H=(gamma+beta)ad*a but using collect throws the error

AttributeError: Can not collect noncommutative symbol

is there any way to do this ? 

[Oscar Benjamin]

You can use factor_nc for noncommutative factorisation:

In [4]: factor_nc(H)
Out[4]: (\gamma + β)⋅a__{\dagger}⋅a

I wanted to link to the docs but it doesn't look like it shows up in
the web docs. You can see the code for it here:
https://github.com/sympy/sympy/blob/d85530009457e4716ed91bfeee995e63fd057b94/sympy/core/exprtools.py#L1404

--
Oscar

[Gerardo Suarez]

Thanks a lot! I've been using a dummy symbol like 

def collect_noncommutative(op,expr):

    dummy= sympy.Symbol('dummy', commutative=True)

    return expr.subs(op,dummy).collect(dummy).subs(dummy,op)

but I knew there had to be a implemented function I just wasn't able to find it