Q:

Get coefficients of a linear pyomo constraint

I would like to obtain the coefficients of a linear constraint c of a pyomo model m. 
For instance, for 
    m= ConcreteModel()
    m.x_1 = Var()
    m.x_2 = Var()
    m.x_3 = Var(within = Integers)
    m.x_4 = Var(within = Integers)
    m.c= Constraint(expr=2*m.x_1 + 5*m.x_2 + m.x_4 <= 2)

I would like to get the array c_coef = [2,5,0,1]. 
The answer to this question  explains how to obtain all variables occurring in a linear constraint and I can easily use this to create the zero-coefficients for variables which don't occur in a constraint.  However, I am struggling with the nonzero-coefficients.  My current approach uses the private attribute _coef, that is c_nzcoef = m.c.body._coef which I probably should not use. 
What would be the proper way to obtain the nonzero coefficients? 

A:

The easiest way to get the coefficients for a linear expression is to make use of the "Canonical Representation" data structure:
from pyomo.repn import generate_canonical_repn
# verify that the expression is linear
if m.c.body.polynominal_degree() == 1:
    repn = generate_canonical_repn(m.c.body)
    for i, coefficient in enumerate(repn.linear or []):
        var = repn.variables[i]

This should be valid for any version of Pyomo from 4.0 through at least 5.3.