Variable
¶

class
myokit.
Variable
(parent, name)¶ Represents a model variable.
Variables should not be created directly, but only via
Component.add_variable()
orVariable.add_variable()
.Each variable has a single defining equation. For state variables, this equation has a derivative on the lefthand side (lhs), for all other variables the lhs of the defining equation is simply the variable’s name.
Variables can be made into state variables using
Variable.promote()
.Variables can be made to represent an externally defined variable using
Variable.set_binding()
. In this case, the right hand side set for a variable will be used as an alternative in situations where the external input is not available.Metadata properties can be accessed via the property
meta
, for examplemodel.meta['key']= 'value'
.
add_variable
(name)¶ Adds a child variable with the given name to this
VarOwner
.

add_variable_allow_renaming
(name)¶ Attempts to add a child variable with the given name to this
VarOwner
, but uses a different name if this causes any conflicts.The new variable’s name will be modified by appending _1, _2, etc. until the conflict is resolved.
This method can be used when symbolically manipulating a model in situations where the exact names are unimportant.
Returns the newly created variable.

binding
()¶ Returns this variable’s binding label or
None
if no binding is set.

can_add_variable
(name, variable_whitelist=None)¶ Returns
True
if a variable can be added to thisVarOwner
under the givenname
.This method is automatically called by
add_variable()
and move_variable(), there is no need to call it before using these methods.To ignore clashes with known variables, a list
variable_whitelist
can be passed in.

code
()¶ Returns this object in
mmt
syntax.

convert_unit
(new_unit, helpers=None)¶ Converts the units this variable is expressed in to
new_unit
.Unit conversion proceeds in the following steps:
 A scaling factor is determined (see below).
 The variable’s RHS is multiplied by this factor
 For state variables, the current state value is also updated.
 All references to the variable in the model equations are divided by the scaling factor.
 If the time variable is updated, all derivative equations are updated.
The scaling factor is determined using
myokit.Unit.conversion_factor(old_unit, new_unit, helpers)()
, whereold_unit
is the current variable unit (as determined byvariable.unit(myokit.UNIT_STRICT)
).For state variables, as with ordinary variables, the
new_unit
should be the unit of the variable itself (and not of its time derivative).Arguments:
new_unit
 The new unit this variable should be in. Given either as a
myokit.Unit
or as a string that can be converted to a unit usingmyokit.parse_unit(new_unit)
. helpers=None
 An optional list of conversion factors, which the method will
attempt to use if the new and old units are incompatible. Each
factor should be specified as a
myokit.Quantity
or something that can be converted to a Quantity e.g. a string1 [uF/cm^2]
.
Note that this method will assume the expression is currently in the unit returned by
Variable.unit()
. It will not check whether the current RHS expression evaluates to the correct units.

count_equations
(const=None, inter=None, state=None, bound=None, deep=False)¶ Returns the number of equations matching the given criteria. See
equations()
for an explanation of the arguments.

count_variables
(const=None, inter=None, state=None, bound=None, deep=False)¶ Returns the number of variables matching the given criteria. See
variables()
for an explanation of the arguments.

demote
()¶ Turns a state variable into an intermediary variable.
This will reset the validation status of the model this variable belongs to.

eq
()¶ Returns this variable’s defining equation.
For state variables this will be an equation for the variable’s derivative. For ordinary variables the equation for the variable’s value is returned.

equations
(const=None, inter=None, state=None, bound=None, deep=False)¶ Creates and returns a filtered iterator over the equations of this object’s variables.
The returned values can be filtered using the following arguments:
const=TrueFalseNone
Set to
True
to return only constants’ equations.False
to exclude all constants and any other value to ignore this check.For a definition of “constant variable” see
variables()
.inter=TrueFalseNone
Set to
True
to return only intermediary variables’ equations,False
to exclude all intermediary variables and any other value to ignore this check.For a definition of “intermediary variable” see
variables()
.state=TrueFalseNone
 Set to
True
to return only state variables’ equations,False
to exclude all state variables and any other value to ignore this check. bound=TrueFalseNone
 Set to
True
to return only bound variables’ equations,False
to exclude all bound variables and any other value to ignore this check. deep=TrueFalse
(by default it’sFalse
) Set to
True
to include the equations of nested variables meeting all other criteria.

eval
()¶ Evaluates this variable’s defining equation and returns the result.

get
(name, class_filter=None)¶ Searches for a variable with the given
qname
and returns it.To return only objects of a certain class, pass it in as
class_filter
.The qnames are specified relative to the
VarOwner
. For example, in a model with a componentina
and a variableina.h
we expect the following results>>> import myokit >>> m = myokit.load_model('example') >>> c = m.get('ina') # Retrieves the component <ina> >>> h = c.get('h') # Retrieves the variable <ina.h> >>> x = c.get('ina.h') # Searches for <ina.ina.h>: KeyError! Traceback (most recent call last): File "<stdin>", line 1, in <module> KeyError: 'ina'

has_equations
(const=None, inter=None, state=None, bound=None, deep=False)¶ Returns True if there are any equations that can be returned by calling :meth:
equations
with the same arguments.

has_variable
(name)¶ Returns True if the given name corresponds to a variable in this object. Accepts both single names
x
and qualified namesx.y
as input.This function performs the same search as
variable
, so in most cases it will be more efficient to callvariable()
in a trycatch block rather than checking for existence explicitly.

has_variables
(const=None, inter=None, state=None, bound=None, deep=False)¶ Returns True if there are any variables that can be returned by calling :meth:
variables
with the same arguments.

indice
()¶ For state variables, this will return their index in the state vector. For all other variables, this will raise an exception.

is_ancestor
(obj)¶ Returns
True
if this object is an ancestor of the givenModelPart`
object
.

is_bound
()¶ Returns
True
if a binding label has been added to this variable.

is_constant
()¶ Returns
True
if this variable is constant.Myokit doesn’t discern between mathematical and physical constants, parameters etc. Anything that doesn’t change during a simulation is termed a constant. Note that this specifically excludes variables bound to external inputs.

is_intermediary
()¶ Returns
True
if this variable is an intermediary variable, i.e. not a constant or a state variable (and not bound to an external variable).

is_labelled
()¶ Returns
True
if a label has been added to this variable.

is_literal
()¶ Returns
True
if this variable’s expression contains only literal values.

is_nested
()¶ Returns
True
if this variable’s parent is another variable.

is_referenced
()¶ Returns
True
if other variables reference this variable’slhs
. For states, this means it only returnsTrue
if other variables depend on this variable’s derivative.

is_state
()¶ Returns
True
if this variable is a state variable.

label
()¶ Returns this variable’s label or
None
if no label is set.

lhs
()¶ Returns the lefthand side of the equation defining this variable.
For state variables this will be a
myokit.Derivative
, for all others this should be amyokit.Name
.

model
()¶ Returns the
myokit.Model
this object belongs to (if set).

move_variable
(variable, new_parent, new_name=None)¶ Moves the given variable to another
VarOwner
new_parent
. In addition, this method can be used to rename variables (either with or without moving them).

name
()¶ Returns this object’s name.

parent
(kind=None)¶ Returns this object’s parent.
If the optional variable
kind
is set, the method will scan until an instance of the requested type is found.

promote
(state_value=0)¶ Turns this variable into a state variable with a current state value given by
state_value
.This will reset the validation status of the model this variable belongs to.

pyfunc
(use_numpy=True, arguments=False)¶ Returns a python function that evaluates this variable as a function of the state variables it depends on.
The argument names used by the returned function will be the variables’ unames, ordered alphabetically.
If the function runs in “numpymode”, which is enabled by default, the vector ready versions of
log
,exp
etc are used and piecewise statements are evaluated using an array ready piecewise function. To disable this functionality, setuse_numpy=False
.To obtain more information about the arguments in the created function handle, set the optional argument
arguments
toTrue
. With this setting, the function will return a tuple(handle, vars)
wherehandle
is the function handle andvars
is a list ofmyokit.LhsExpression
objects in the same order as the function arguments.

qname
(hide=None)¶ Returns this object’s fully qualified name. That is, an object named
y
with a parent namedx
will return the stringx.y
. Ify
has a childz
its qname will bex.y.z
.If the optional argument
hide
is set to this object’s parent the parent qualifier will be omitted.

refs_by
(state_refs=False)¶ Returns an iterator over the set of
Variables
that refer to this variable in their defining equation.Note that only references to this variable’s defining
LhsExpression
(i.e. the one returned bylhs()
) are returned. For a state variablex
, this means the returned result contains all variables referring todot(x)
. To get an iterator over the variables referring tox
instead, add the optional attributestate_refs=True
. For nonstate variables this setting will trigger anException
.

refs_to
(state_refs=False)¶ Returns an iterator over the set of
Variables
that this variable’s defining equation refers to.By default, this will _not_ include references to a state variable’s value. To obtain a list of state variables whose value is referenced, use
state_refs=True
.

remove_variable
(variable, recursive=False)¶ Removes the given variable from this
VarOwner
and from the model.If
recursive
isTrue
, any child variables will be deleted as well.

rename
(new_name)¶ Renames this variable.

rhs
()¶ Returns the righthand side of the equation defining this variable.
For state variables this will be the expression for their derivative, for all others an expression for their value is returned.

set_binding
(binding)¶ Adds a unique binding label to this variable, indicating it can be used as an entry point for external inputs.
To remove a binding, call
set_binding(None)
.Adding or removing binding labels resets the model’s validation status.

set_label
(label=None)¶ Adds a unique
label
for this variable, indicated that its value can be read by external users.To remove a label call
set_label(None)
.

set_rhs
(rhs)¶ Changes the expression for this variable’s right hand side (rhs).
For state variables, this updates their derivative. For all others this will update the expression for their value.
Expressions can be specified using
myokit.Expression
objects, but floats or strings that can be parsed into expressions are also allowed:# Ways of setting x = 2 x.set_rhs(myokit.Number(2)) x.set_rhs(2) x.set_rhs(2.0) x.set_rhs('2') # Ways of setting x = 1 + y x.set_rhs(myokit.Plus(myokit.Number(1), myokit.Name(y))) x.set_rhs('1 + y')
Calling set_rhs will reset the validation status of the model this variable belongs to.

set_state_value
(value)¶ If this variable is a state variable, its current value will be updated. For all other variables this raises an exception.

set_unit
(unit=None)¶ Changes this variable’s unit. The unit can be set to any valid Unit object, or
None
to remove the unit.

state_value
()¶ For state variables, this will return their current value. For all other variables, this will raise an exception.

uname
()¶ Returns a globally unique name for this object.

unit
(mode=1)¶ Returns the unit specified for this variable.
If no unit was set and
mode
ismyokit.UNIT_TOLERANT
, then None is returned. Inmyokit.UNIT_STRICT
mode the valuemyokit.units.dimensionless
is returned in this case.Variables’ units are set using
Variable.set_unit()
or using thein
keyword in.mmt
files. The unit set for a variable is the unit this variable’s expression _should_ have. This allows expressions to be validated by computing the resulting unit of an expression and comparing it with the value set for the variable.

validate
()¶ Attempts to check this variable’s validity, raises errors if it isn’t.

value
()¶ Will return the value of this variable’s defining righthand side expression.

var
(name)¶ Searches for the given variable and returns it if found. Accepts both single names
x
and qualified namesx.y
as input.

variables
(const=None, inter=None, state=None, bound=None, deep=False, sort=False)¶ Creates and returns a filtered iterator over the contained variables.
The returned values can be filtered using the following arguments:
const=TrueFalseNone
Constants are defined as variables that do not depend on state variables or derivatives. In other words, any variable whose value can be determined before starting an ODE solving routine.
Set to
True
to return only constants,False
to exclude all constants and any other value to ignore this check.inter=TrueFalseNone
Intermediary variables are those variables that are not constant but are not part of the state. In other words, intermediary variables are the variables that need to be calculated at every step of an ODE solving routine before calculating the ODE derivatives and updating the state.
Set to
True
to return only intermediary variables,False
to exclude all intermediary variables and any other value to ignore this check.state=TrueFalseNone
 Set to True to return only state variables, False to exclude all state variables and any other value to ignore this check.
bound=TrueFalseNone
 Set to True to return only variables bound to an external value, False to exclude all bound variables and any other value to forgo this check.
deep=TrueFalse
(by default it’sFalse
) Set to True to return nested variables meeting all other criteria.
sort=TrueFalse
(by default it’sFalse
) Set to True to return the variables in a consistent order. (Note that this does _not_ mean alphabetical sorting of all variables, just that the order is consistent between calls!)
