# Source code for elastica.wrappers.constraints

```
__doc__ = """
Constraints
-----------
Provides the constraints interface to enforce displacement boundary conditions (see `boundary_conditions.py`).
"""
from elastica.boundary_conditions import FreeRod
[docs]class Constraints:
"""
The Constraints class is a wrapper for enforcing displacement boundary conditions.
To enforce boundary conditions on rod-like objects, the simulator class
must be derived from Constraints class.
Attributes
----------
_constraints: list
List of boundary condition classes defined for rod-like objects.
"""
def __init__(self):
self._constraints = []
super(Constraints, self).__init__()
[docs] def constrain(self, system):
"""
This method enforces a displacement boundary conditions to the relevant user-defined
system or rod-like object. You must input the system or rod-like
object that you want to enforce boundary condition on.
Parameters
----------
system: object
System is a rod-like object.
Returns
-------
"""
sys_idx = self._get_sys_idx_if_valid(system)
# Create _Constraint object, cache it and return to user
_constraint = _Constraint(sys_idx)
self._constraints.append(_constraint)
return _constraint
def _finalize(self):
# From stored _Constraint objects, instantiate the boundary conditions
# inplace : https://stackoverflow.com/a/1208792
# dev : the first index stores the rod index to apply the boundary condition
# to. Technically we can use another array but it its one more book-keeping
# step. Being lazy, I put them both in the same array
self._constraints[:] = [
(constraint.id(), constraint(self._systems[constraint.id()]))
for constraint in self._constraints
]
# Sort from lowest id to highest id for potentially better memory access
# _constraints contains list of tuples. First element of tuple is rod number and
# following elements are the type of boundary condition such as
# [(0, FreeRod, OneEndFixedRod), (1, HelicalBucklingBC), ... ]
# Thus using lambda we iterate over the list of tuples and use rod number (x[0])
# to sort constraints.
self._constraints.sort(key=lambda x: x[0])
# At t=0.0, constrain all the boundary conditions (for compatability with
# initial conditions)
# TODO: you may need to change naming of _callBC
self._callBC(time=0.0)
# TODO: same as above naming of _callBC function
def _callBC(self, time, *args, **kwargs):
for sys_id, constraint in self._constraints:
constraint.constrain_values(self._systems[sys_id], time, *args, **kwargs)
constraint.constrain_rates(self._systems[sys_id], time, *args, **kwargs)
class _Constraint:
"""
Constraint wrapper private class
Attributes
----------
_sys_idx: int
_bc_cls: list
*args
Variable length argument list.
**kwargs
Arbitrary keyword arguments.
"""
def __init__(self, sys_idx: int):
"""
Parameters
----------
sys_idx: int
"""
self._sys_idx = sys_idx
self._bc_cls = None
self._args = ()
self._kwargs = {}
def using(self, bc_cls, *args, **kwargs):
"""
This method is a wrapper to set which boundary condition class is used to
enforce boundary condition from user defined rod-like objects.
Parameters
----------
bc_cls : object
User defined boundary condition class.
*args
Variable length argument list
**kwargs
Arbitrary keyword arguments.
Returns
-------
"""
assert issubclass(
bc_cls, FreeRod
), "{} is not a valid boundary condition. Did you forget to derive from FreeRod?".format(
bc_cls
)
self._bc_cls = bc_cls
self._args = args
self._kwargs = kwargs
return self
def id(self):
return self._sys_idx
def __call__(self, rod, *args, **kwargs):
""" Constructs a constraint after checks
Parameters
----------
args
kwargs
Returns
-------
"""
if not self._bc_cls:
raise RuntimeError(
"No boundary condition provided to constrain rod"
"id {0} at {1}, but a BC was intended. Did you"
"forget to call the `using` method?".format(self.id(), rod)
)
# If there is position, director in kwargs, deal with it first
# Returns None if not found
pos_indices = self._kwargs.pop(
"constrained_position_idx", None
) # calculate position indices as a tuple
director_indices = self._kwargs.pop(
"constrained_director_idx", None
) # calculate director indices as a tuple
# If pos_indices is not None, construct list else empty list
# IMPORTANT : do copy for memory-safe operations
positions = (
[rod.position_collection[..., idx].copy() for idx in pos_indices]
if pos_indices
else []
)
directors = (
[rod.director_collection[..., idx].copy() for idx in director_indices]
if director_indices
else []
)
try:
return self._bc_cls(*positions, *directors, *self._args, **self._kwargs)
except (TypeError, IndexError):
raise TypeError(
"Unable to construct boundary condition class. Note that:\n"
"1. Any rod properties needed should be placed first\n"
"in the boundary_condition __init__ like so (pos_one, pos_two, <other_args>)\n"
"2. Number of requested position and directors such as (1, 2) should match\n"
"the __init__ method. eg MyBC.__init__(pos_one, director_one, director_two)\n"
"should have the `using` call as .using(MyBC, positions=(1,), directors=(1,-1))\n"
)
```