Source code for ufl.core.expr

"""This module defines the ``Expr`` class, the superclass for all expression tree node types in UFL.

NB: A note about other operators not implemented here:

More operators (special functions) on ``Expr`` instances are defined in
``exproperators.py``, as well as the transpose ``A.T`` and spatial derivative
``a.dx(i)``.
This is to avoid circular dependencies between ``Expr`` and its subclasses.
"""
# Copyright (C) 2008-2016 Martin Sandve Aln├Žs
#
# This file is part of UFL (https://www.fenicsproject.org)
#
# SPDX-License-Identifier:    LGPL-3.0-or-later
#
# Modified by Anders Logg, 2008
# Modified by Massimiliano Leoni, 2016

import warnings

from ufl.core.ufl_type import UFLType, update_ufl_type_attributes


[docs]class Expr(object, metaclass=UFLType): """Base class for all UFL expression types. *Instance properties* Every ``Expr`` instance will have certain properties. The most important ones are ``ufl_operands``, ``ufl_shape``, ``ufl_free_indices``, and ``ufl_index_dimensions`` properties. Expressions are immutable and hashable. *Type traits* The ``Expr`` API defines a number of type traits that each subclass needs to provide. Most of these are specified indirectly via the arguments to the ``ufl_type`` class decorator, allowing UFL to do some consistency checks and automate most of the traits for most types. Type traits are accessed via a class or instance object of the form ``obj._ufl_traitname_``. See the source code for description of each type trait. *Operators* Some Python special functions are implemented in this class, some are implemented in subclasses, and some are attached to this class in the ``ufl_type`` class decorator. *Defining subclasses* To define a new expression class, inherit from either ``Terminal`` or ``Operator``, and apply the ``ufl_type`` class decorator with suitable arguments. See the docstring of ``ufl_type`` for details on its arguments. Looking at existing classes similar to the one you wish to add is a good idea. Looking through the comments in the ``Expr`` class and ``ufl_type`` to understand all the properties that may need to be specified is also a good idea. Note that many algorithms in UFL and form compilers will need handlers implemented for each new type::. .. code-block:: python @ufl_type() class MyOperator(Operator): pass *Type collections* All ``Expr`` subclasses are collected by ``ufl_type`` in global variables available via ``Expr``. *Profiling* Object creation statistics can be collected by doing .. code-block:: python Expr.ufl_enable_profiling() # ... run some code initstats, delstats = Expr.ufl_disable_profiling() Giving a list of creation and deletion counts for each typecode. """ # --- Each Expr subclass must define __slots__ or _ufl_noslots_ at # --- the top --- # This is to freeze member variables for objects of this class and # save memory by skipping the per-instance dict. __slots__ = ("_hash", "__weakref__") # _ufl_noslots_ = True # --- Basic object behaviour --- def __getnewargs__(self): """Get newargs tuple. The tuple returned here is passed to as args to cls.__new__(cls, *args). This implementation passes the operands, which is () for terminals. May be necessary to override if __new__ is implemented in a subclass. """ return self.ufl_operands def __init__(self): """Initialise.""" self._hash = None # This shows the principal behaviour of the hash function attached # in ufl_type: # def __hash__(self): # if self._hash is None: # self._hash = self._ufl_compute_hash_() # return self._hash # --- Type traits are added to subclasses by the ufl_type class # --- decorator --- # Note: Some of these are modified after the Expr class definition # because Expr is not defined yet at this point. Note: Boolean # type traits that categorize types are mostly set to None for # Expr but should be True or False for any non-abstract type. # A reference to the UFL class itself. This makes it possible to # do type(f)._ufl_class_ and be sure you get the actual UFL class # instead of a subclass from another library. _ufl_class_ = None # The handler name. This is the name of the handler function you # implement for this type in a multifunction. _ufl_handler_name_ = "expr" # Number of operands, "varying" for some types, or None if not # applicable for abstract types. _ufl_num_ops_ = None # Type trait: If the type is a literal. _ufl_is_literal_ = None # Type trait: If the type is classified as a 'terminal modifier', # for form compiler use. _ufl_is_terminal_modifier_ = None # Type trait: If the type is a shaping operator. Shaping # operations include indexing, slicing, transposing, i.e. not # introducing computation of a new value. _ufl_is_shaping_ = False # Type trait: If the type is in reference frame. _ufl_is_in_reference_frame_ = None # Type trait: If the type is a restriction to a geometric entity. _ufl_is_restriction_ = None # Type trait: If the type is evaluation in a particular way. _ufl_is_evaluation_ = None # Type trait: If the type is a differential operator. _ufl_is_differential_ = None # Type trait: If the type is purely scalar, having no shape or # indices. _ufl_is_scalar_ = None # Type trait: If the type never has free indices. _ufl_is_index_free_ = False # --- All subclasses must define these object attributes --- # Each subclass of Expr is checked to have these properties in # ufl_type _ufl_required_properties_ = ( # A tuple of operands, all of them Expr instances. "ufl_operands", # A tuple of ints, the value shape of the expression. "ufl_shape", # A tuple of free index counts. "ufl_free_indices", # A tuple providing the int dimension for each free index. "ufl_index_dimensions", ) # Each subclass of Expr is checked to have these methods in # ufl_type # FIXME: Add more and enable all _ufl_required_methods_ = ( # To compute the hash on demand, this method is called. "_ufl_compute_hash_", # The data returned from this method is used to compute the # signature of a form "_ufl_signature_data_", # The == operator must be implemented to compare for identical # representation, used by set() and dict(). The __hash__ # operator is added by ufl_type. "__eq__", # To reconstruct an object of the same type with operands or # properties changed. "_ufl_expr_reconstruct_", # Implemented in Operator and Terminal so this should never fail "ufl_domains", # "ufl_cell", # "ufl_domain", # "__str__", # "__repr__", ) # --- Global variables for collecting all types --- # A global dict mapping language_operator_name to the type it # produces _ufl_language_operators_ = {} # List of all terminal modifier types _ufl_terminal_modifiers_ = [] # --- Mechanism for profiling object creation and deletion --- # Backup of default init and del _ufl_regular__init__ = __init__ def _ufl_profiling__init__(self): """Replacement constructor with object counting.""" Expr._ufl_regular__init__(self) Expr._ufl_obj_init_counts_[self._ufl_typecode_] += 1 def _ufl_profiling__del__(self): """Replacement destructor with object counting.""" Expr._ufl_obj_del_counts_[self._ufl_typecode_] -= 1
[docs] @staticmethod def ufl_enable_profiling(): """Turn on the object counting mechanism and reset counts to zero.""" Expr.__init__ = Expr._ufl_profiling__init__ setattr(Expr, "__del__", Expr._ufl_profiling__del__) for i in range(len(Expr._ufl_obj_init_counts_)): Expr._ufl_obj_init_counts_[i] = 0 Expr._ufl_obj_del_counts_[i] = 0
[docs] @staticmethod def ufl_disable_profiling(): """Turn off the object counting mechanism. Return object init and del counts.""" Expr.__init__ = Expr._ufl_regular__init__ delattr(Expr, "__del__") return (Expr._ufl_obj_init_counts_, Expr._ufl_obj_del_counts_)
# === Abstract functions that must be implemented by subclasses === # --- Functions for reconstructing expression --- def _ufl_expr_reconstruct_(self, *operands): """Return a new object of the same type with new operands.""" raise NotImplementedError(self.__class__._ufl_expr_reconstruct_) # --- Functions for geometric properties of expression ---
[docs] def ufl_domains(self): """Return all domains this expression is defined on.""" warnings.warn( "Expr.ufl_domains() is deprecated, please use extract_domains(expr) instead.", DeprecationWarning, ) from ufl.domain import extract_domains return extract_domains(self)
[docs] def ufl_domain(self): """Return the single unique domain this expression is defined on, or throw an error.""" warnings.warn( "Expr.ufl_domain() is deprecated, please use extract_unique_domain(expr) instead.", DeprecationWarning, ) from ufl.domain import extract_unique_domain return extract_unique_domain(self)
# --- Functions for float evaluation ---
[docs] def evaluate(self, x, mapping, component, index_values): """Evaluate expression at given coordinate with given values for terminals.""" raise ValueError(f"Symbolic evaluation of {self._ufl_class_.__name__} not available.")
def _ufl_evaluate_scalar_(self): if self.ufl_shape or self.ufl_free_indices: raise TypeError("Cannot evaluate a nonscalar expression to a scalar value.") return self(()) # No known x def __float__(self): """Try to evaluate as scalar and cast to float.""" try: v = float(self._ufl_evaluate_scalar_()) except Exception: v = NotImplemented return v def __complex__(self): """Try to evaluate as scalar and cast to complex.""" try: v = complex(self._ufl_evaluate_scalar_()) except TypeError: v = NotImplemented return v def __bool__(self): """By default, all Expr are nonzero/False.""" return True def __nonzero__(self): """By default, all Expr are nonzero/False.""" return self.__bool__() @staticmethod def _ufl_coerce_(value): """Convert any value to a UFL type.""" # Quick skip for most types if isinstance(value, Expr): return value # Conversion from non-ufl types # (the _ufl_from_*_ functions are attached to Expr by ufl_type) ufl_from_type = "_ufl_from_{0}_".format(value.__class__.__name__) return getattr(Expr, ufl_from_type)(value) # if hasattr(Expr, ufl_from_type): # return getattr(Expr, ufl_from_type)(value) # Fail gracefully if no valid type conversion found # raise TypeError("Cannot convert a {0.__class__.__name__} to UFL type.".format(value)) # --- Special functions for string representations --- # All subclasses must implement _ufl_signature_data_ def _ufl_signature_data_(self, renumbering): """Return data that uniquely identifies form compiler relevant aspects of this object.""" raise NotImplementedError(self.__class__._ufl_signature_data_) # All subclasses must implement __repr__ def __repr__(self): """Return string representation this object can be reconstructed from.""" raise NotImplementedError(self.__class__.__repr__) # All subclasses must implement __str__ def __str__(self): """Return pretty print string representation of this object.""" raise NotImplementedError(self.__class__.__str__) def _ufl_err_str_(self): """Return a short string to represent this Expr in an error message.""" return f"<{self._ufl_class_.__name__} id={id(self)}>" # --- Special functions used for processing expressions --- def __eq__(self, other): """Checks whether the two expressions are represented the exact same way. This does not check if the expressions are mathematically equal or equivalent! Used by sets and dicts. """ raise NotImplementedError(self.__class__.__eq__) def __len__(self): """Length of expression. Used for iteration over vector expressions.""" s = self.ufl_shape if len(s) == 1: return s[0] raise NotImplementedError("Cannot take length of non-vector expression.") def __iter__(self): """Iteration over vector expressions.""" for i in range(len(self)): yield self[i] def __floordiv__(self, other): """UFL does not support integer division.""" raise NotImplementedError(self.__class__.__floordiv__) def __pos__(self): """Unary + is a no-op.""" return self def __round__(self, n=None): """Round to nearest integer or to nearest nth decimal.""" try: val = float(self._ufl_evaluate_scalar_()) val = round(val, n) except TypeError: val = complex(self._ufl_evaluate_scalar_()) val = round(val.real, n) + round(val.imag, n) * 1j except TypeError: val = NotImplemented return val
# Initializing traits here because Expr is not defined in the class # declaration Expr._ufl_class_ = Expr # Update Expr with metaclass properties (e.g. typecode or handler name) # Explicitly done here instead of using `@ufl_type` to avoid circular imports. update_ufl_type_attributes(Expr)
[docs]def ufl_err_str(expr): """Return a UFL error string.""" if hasattr(expr, "_ufl_err_str_"): return expr._ufl_err_str_() else: return repr(expr)