pymel.core.effects.dynExpression

dynExpression(*args, **kwargs)

This command describes an expression that belongs to the specified particle shape. The expression is a block of code of unlimited length with a C-like syntax that can perform conversions, mathematical operations, and logical decision making on any numeric attribute(s) or per-particle attribute(s) in the scene. One expression can read and alter any number of these attributes. Every particle shape in your scene has three expressions, one for the runtimeBeforeDynamics, one for the runtimeAfterDynamics and one for creation time. The create expression gets executed for every particle in the object whose age is 0.0. The runtime expression gets executed for each particle with an age greater then 0.0. Unlike expressions created with the expressioncommand, particle expressions always exist and are a part of the owning particle object’s shape. They default to empty strings, but they are always there. Because of this, there is no need to use the ‘-e’ flag. Every call to the dynExpression command is considered an edit by default. Per-particle attributes are those attributes of a particle shape that have a potentially different value for each particle in the object. Examples of these include positionand velocity. If this command is being sent by the command line or in a script, then the user should be sure to embed escaped newlines (n), tabs (t) for clarity when reading them in the expression editor. Also, quotes in an expression must be escaped (”) so that they are not confused by the system as the end of your string. When using the expression editor, these characters are escaped for you unless they are already within quotes. This type of expression is executed during the evaluation of the dynamics. If an output of the expression is requested before that, then the dynamics will be force to compute at that time. If dynamics is disabled, then these expressions will have no effect.

Flags:

Long name (short name)		Argument Types	Properties
creation (c)		bool
	Tells the command that the string passed will be a creation expression for the particle shape. This means that this expression will be executed when a particle is emitted or at the beginning of the scene for existing particles.
name (n)		unicode
runtime (r)		bool
runtimeAfterDynamics (rad)		bool
	Tells the command that the string passed will be a runtime expression for the particle shape. This expression will be executed after dynamics whenever a particle’s age is greater then zero (0).Flag can appear in Create mode of commandFlag can have multiple arguments, passed either as a tuple or a list.
runtimeBeforeDynamics (rbd)		bool
	Tells the command that the string passed will be a runtime expression for the particle shape. This expression will be executed before dynamics whenever a particle’s age is greater then zero (0).
string (s)		unicode
	Set the expression string. This is queriable with the -q/query flag and the -rbd/runtimeBeforeDynamics, the -rab/runtimeAfterDynamics or the -c/creation flag.

Derived from mel command maya.cmds.dynExpression

Example:

import pymel.core as pm

pm.dynExpression( 'particleShape1', s='rgbPP = "" 1, 0, 0 ""', c=1 )

# This expression tells particleShape1 that whenever new particles are
# created for this object, then their color should start out as "" 1, 0, 0 "",
# which is red.

pm.dynExpression( 'particleShape1', s='rgbPP = rgbPP * .9;', rbd=1 )

# This sets the runtime before dynamics expression for rgbPP.  When a particle is
# first "born", its color will be red from the previous example.  Every other frame after
# that, its color is reduced by 10 percent each time the expression is executed.

pm.dynExpression( 'particleShape1', s='rgbPP = rgbPP * .9;', rad=1 )

# This sets the runtime after dynamics expression for rgbPP.  When a particle is
# first "born", its color will be red from the previous example.  Every other frame after
# that, its color is reduced by 10 percent each time the expression is executed.