Expression Manipulation (Docstrings)¶
msubs¶

sympy.physics.mechanics.
msubs
(expr, *sub_dicts, **kwargs)[source]¶ A custom subs for use on expressions derived in physics.mechanics.
Traverses the expression tree once, performing the subs found in sub_dicts. Terms inside
Derivative
expressions are ignored:>>> from sympy.physics.mechanics import dynamicsymbols, msubs >>> x = dynamicsymbols('x') >>> msubs(x.diff() + x, {x: 1}) Derivative(x(t), t) + 1
Note that sub_dicts can be a single dictionary, or several dictionaries:
>>> x, y, z = dynamicsymbols('x, y, z') >>> sub1 = {x: 1, y: 2} >>> sub2 = {z: 3, x.diff(): 4} >>> msubs(x.diff() + x + y + z, sub1, sub2) 10
If smart=True (default False), also checks for conditions that may result in
nan
, but if simplified would yield a valid expression. For example:>>> from sympy import sin, tan >>> (sin(x)/tan(x)).subs(x, 0) nan >>> msubs(sin(x)/tan(x), {x: 0}, smart=True) 1
It does this by first replacing all
tan
withsin/cos
. Then each node is traversed. If the node is a fraction, subs is first evaluated on the denominator. If this results in 0, simplification of the entire fraction is attempted. Using this selective simplification, only subexpressions that result in 1/0 are targeted, resulting in faster performance.
find_dynamicsymbols¶

sympy.physics.mechanics.
find_dynamicsymbols
(expression, exclude=None, reference_frame=None)[source]¶ Find all dynamicsymbols in expression.
If the optional
exclude
kwarg is used, only dynamicsymbols not in the iterableexclude
are returned. If we intend to apply this function on a vector, the optional ‘’reference_frame’’ is also used to inform about the corresponding frame with respect to which the dynamic symbols of the given vector is to be determined. Parameters
expression : sympy expression
exclude : iterable of dynamicsymbols, optional
reference_frame : ReferenceFrame, optional
The frame with respect to which the dynamic symbols of the given vector is to be determined.
Examples
>>> from sympy.physics.mechanics import dynamicsymbols, find_dynamicsymbols >>> from sympy.physics.mechanics import ReferenceFrame >>> x, y = dynamicsymbols('x, y') >>> expr = x + x.diff()*y >>> find_dynamicsymbols(expr) {x(t), y(t), Derivative(x(t), t)} >>> find_dynamicsymbols(expr, exclude=[x, y]) {Derivative(x(t), t)} >>> a, b, c = dynamicsymbols('a, b, c') >>> A = ReferenceFrame('A') >>> v = a * A.x + b * A.y + c * A.z >>> find_dynamicsymbols(v, reference_frame=A) {a(t), b(t), c(t)}