scipy.optimize.minimize(method=’trust-constr’) doesn't terminate on xtol condition

Question

I have set up an optimization problem with linear equality constraints as follows

sol0 = minimize(objective, x0, args=mock_df, method='trust-constr',
                bounds=bnds, constraints=cons,
                options={'maxiter': 250, 'verbose': 3})

The objective is a weighted sum functions, whose coefficients/weights are to be optimized to make it minimized. As I have boundaries on the coefficients as well as constraints, I used the trust-constr method within scipy.optimize.minimize.

The minimization works out, but I do not understand the termination criteria. According to the trust-constr documentation it should terminate on xtol

The algorithm will terminate when tr_radius < xtol, where tr_radius is the radius of the trust region used in the algorithm. Default is 1e-8.

However, the verbose output shows, that the termination is indeed triggered by the barrier_tol parameter, as you can see in the listing below

| niter |f evals|CG iter|  obj func   |tr radius |   opt    |  c viol  | penalty  |barrier param|CG stop|
|-------|-------|-------|-------------|----------|----------|----------|----------|-------------|-------|
C:\ProgramData\Anaconda3\lib\site-packages\scipy\optimize\_trustregion_constr\projections.py:182: UserWarning: Singular Jacobian matrix. Using SVD decomposition to perform the factorizations.
  warn('Singular Jacobian matrix. Using SVD decomposition to ' +
|   1   |  31   |   0   | -4.4450e+02 | 1.00e+00 | 7.61e+02 | 5.00e-01 | 1.00e+00 |  1.00e-01   |   0   |
C:\ProgramData\Anaconda3\lib\site-packages\scipy\optimize\_hessian_update_strategy.py:187: UserWarning: delta_grad == 0.0. Check if the approximated function is linear. If the function is linear better results can be obtained by defining the Hessian as zero instead of using quasi-Newton approximations.
  'approximations.', UserWarning)
|   2   |  62   |   1   | -2.2830e+03 | 6.99e+00 | 3.64e+02 | 7.28e-01 | 1.00e+00 |  1.00e-01   |   2   |
|   3   |  93   |   2   | -9.7651e+03 | 3.42e+01 | 5.52e+01 | 5.33e+00 | 1.00e+00 |  1.00e-01   |   2   |
|   4   |  124  |  26   | -4.9999e+03 | 3.42e+01 | 8.23e+01 | 9.29e-01 | 3.48e+16 |  1.00e-01   |   1   |
|   5   |  155  |  50   | -4.1486e+03 | 3.42e+01 | 5.04e+01 | 2.08e-01 | 3.48e+16 |  1.00e-01   |   1   |
...
|  56   | 1674  | 1127  | -1.6146e+03 | 1.77e-08 | 4.49e+00 | 3.55e-15 | 3.66e+33 |  1.00e-01   |   1   |
|  57   | 1705  | 1151  | -1.6146e+03 | 1.77e-09 | 4.49e+00 | 3.55e-15 | 3.66e+33 |  1.00e-01   |   1   |
|  58   | 1736  | 1151  | -1.6146e+03 | 1.00e+00 | 4.42e+00 | 3.55e-15 | 1.00e+00 |  2.00e-02   |   0   |
|  59   | 1767  | 1175  | -1.6146e+03 | 1.00e-01 | 4.42e+00 | 3.55e-15 | 1.00e+00 |  2.00e-02   |   1   |
|  60   | 1798  | 1199  | -1.6146e+03 | 1.00e-02 | 4.42e+00 | 3.55e-15 | 1.00e+00 |  2.00e-02   |   1   |
...
|  66   | 1984  | 1343  | -1.6146e+03 | 1.00e-08 | 4.42e+00 | 3.55e-15 | 1.00e+00 |  2.00e-02   |   1   |
|  67   | 2015  | 1367  | -1.6146e+03 | 1.00e-09 | 4.42e+00 | 3.55e-15 | 1.00e+00 |  2.00e-02   |   1   |
|  68   | 2046  | 1367  | -1.6146e+03 | 1.00e+00 | 4.36e+00 | 3.55e-15 | 1.00e+00 |  4.00e-03   |   0   |
|  69   | 2077  | 1391  | -1.6146e+03 | 1.00e-01 | 4.36e+00 | 3.55e-15 | 1.00e+00 |  4.00e-03   |   1   |
...
|  77   | 2325  | 1583  | -1.6146e+03 | 1.00e-09 | 4.36e+00 | 3.55e-15 | 1.00e+00 |  4.00e-03   |   1   |
|  78   | 2356  | 1583  | -1.6146e+03 | 1.00e+00 | 4.35e+00 | 3.55e-15 | 1.00e+00 |  8.00e-04   |   0   |
|  79   | 2387  | 1607  | -1.6146e+03 | 1.00e-01 | 4.35e+00 | 3.55e-15 | 1.00e+00 |  8.00e-04   |   1   |
...
|  87   | 2635  | 1799  | -1.6146e+03 | 1.00e-09 | 4.35e+00 | 3.55e-15 | 1.00e+00 |  8.00e-04   |   1   |
|  88   | 2666  | 1799  | -1.6146e+03 | 1.00e+00 | 4.34e+00 | 3.55e-15 | 1.00e+00 |  1.60e-04   |   0   |
|  89   | 2697  | 1823  | -1.6146e+03 | 1.00e-01 | 4.34e+00 | 3.55e-15 | 1.00e+00 |  1.60e-04   |   1   |
...
|  97   | 2945  | 2015  | -1.6146e+03 | 1.00e-09 | 4.34e+00 | 3.55e-15 | 1.00e+00 |  1.60e-04   |   1   |
|  98   | 2976  | 2015  | -1.6146e+03 | 1.00e+00 | 4.34e+00 | 3.55e-15 | 1.00e+00 |  3.20e-05   |   0   |
|  99   | 3007  | 2039  | -1.6146e+03 | 1.00e-01 | 4.34e+00 | 3.55e-15 | 1.00e+00 |  3.20e-05   |   1   |
...
|  167  | 5053  | 3527  | -1.6146e+03 | 1.00e-07 | 1.35e+01 | 2.12e-11 | 1.00e+00 |  2.05e-09   |   1   |
|  168  | 5084  | 3551  | -1.6146e+03 | 1.00e-08 | 1.35e+01 | 2.12e-11 | 1.00e+00 |  2.05e-09   |   1   |
|  169  | 5115  | 3575  | -1.6146e+03 | 1.00e-09 | 1.35e+01 | 2.12e-11 | 1.00e+00 |  2.05e-09   |   1   |
`xtol` termination condition is satisfied.
Number of iterations: 169, function evaluations: 5115, CG iterations: 3575, optimality: 1.35e+01, constraint violation: 2.12e-11, execution time: 3.8e+02 s.

It is obvious that once, tr_radius < xtol, the tr_radius is reset to its default value 1 and the barrier param is reduced. Once barrier param < barrier_tol (i.e. 1e-8) and tr_radius < xtol, the optimization terminates successfully. The documentation says regarding barrier_tol

When inequality constraints are present the algorithm will terminate only when the barrier parameter is less than barrier_tol.

which would explain the behaviour in case of inequality constraints, but all my constraints are equality constraints defined as dictionary

con0 = {'type': 'eq', 'fun': constraint0}

Is anyone deep enough into trust-constr to explain this to me?

Answer 1

It is linked to an internal conversion of variable boundaries to inequality constraints via the PreparedConstraints class and the initial_constraints_as_canonical function in the function _minimize_trustregion_constr within minimize(method='trust-constr').

The source code, where this is defined can be found in scipy/scipy/optimize/_trustregion_constr/minimize_trustregion_constr.py

The responsible code lines are

if bounds is not None:
    if sparse_jacobian is None:
        sparse_jacobian = True
    prepared_constraints.append(PreparedConstraint(bounds, x0,
                                                   sparse_jacobian))

where the algorithm appends defined variable boundaries bounds as PreparedConstraint to the list of the originally defined constraints already prepared in prepared_constraints. The successing lines

# Concatenate initial constraints to the canonical form.
c_eq0, c_ineq0, J_eq0, J_ineq0 = initial_constraints_as_canonical(
    n_vars, prepared_constraints, sparse_jacobian)

convert each boundary to two inequality constraints (x > lb and x < ub) and returns therefore additional contraints in an amount twice of the number of boundaries.

_minimize_trustregion_constr then detects those unequality constraints and correctly chooses thus the algorithm tr_interior_point

# Choose appropriate method
if canonical.n_ineq == 0:
    method = 'equality_constrained_sqp'
else:
    method = 'tr_interior_point'

In the following, the problem is treated as a problem originally containing inequality constraints and is thus, correctly terminating on the xtol condition AND the barrier_parameter condition as decribed in the question.