Syntax Guide

This page summarizes the syntax of Scenic (excluding syntax inherited from Python). For more details, click the links for individual language constructs to go to the corresponding section of the Language Reference.

Primitive Data Types

Booleans	expressing truth values
Scalars	representing distances, angles, etc. as floating-point numbers
Vectors	representing positions and offsets in space
Headings	representing orientations in space
Vector Fields	associating an orientation (i.e. a heading) to each point in space
Regions	representing sets of points in space

Distributions

Range(low, high)	uniformly-distributed real number in the interval
DiscreteRange(low, high)	uniformly-distributed integer in the (fixed) interval
Normal(mean, stdDev)	normal distribution with the given mean and standard deviation
TruncatedNormal(mean, stdDev, low, high)	normal distribution truncated to the given window
Uniform(value, ...)	uniform over a finite set of values
Discrete({value: weight, ...})	discrete with given values and weights
Point (in | on) region	uniformly-distributed Point in a region

Statements

Compound Statements

Syntax	Meaning
class name[(superclass)]:	Defines a Scenic class.
behavior name(arguments):	Defines a dynamic behavior.
monitor name:	Defines a monitor.
scenario name(arguments):	Defines a modular scenario.
try: ... interrupt when boolean:	Run code with interrupts inside a dynamic behavior or modular scenario.

Simple Statements

Syntax	Meaning
model name	Select the world model.
import module	Import a Scenic or Python module.
param name = value, ...	Define global parameters of the scenario.
require boolean	Define a hard requirement.
require[number] boolean	Define a soft requirement.
require (always | eventually) boolean	Define a dynamic hard requirement.
terminate when boolean	Define a termination condition.
terminate after scalar (seconds | steps)	Set the scenario to terminate after a given amount of time.
mutate identifier, ... [by number]	Enable mutation of the given list of objects.
record [initial | final] value as name	Save a value at every time step or only at the start/end of the simulation.

Dynamic Statements

These statements can only be used inside a dynamic behavior, monitor, or compose block of a modular scenario.

Syntax	Meaning
take action, ...	Take the action(s) specified.
wait	Take no actions this time step.
terminate	Immediately end the scenario.
do behavior/scenario, ...	Run one or more sub-behaviors/sub-scenarios until they complete.
do behavior/scenario, ... until boolean	Run sub-behaviors/scenarios until they complete or a condition is met.
do behavior/scenario, ... for scalar (seconds | steps)	Run sub-behaviors/scenarios for (at most) a specified period of time.
do choose behavior/scenario, ...	Run one choice of sub-behavior/scenario whose preconditions are satisfied.
do shuffle behavior/scenario, ...	Run several sub-behaviors/scenarios in a random order, satisfying preconditions.
abort	Break out of the current try-interrupt statement.
override object specifier, ...	Override properties of an object for the duration of the current scenario.

Objects

The syntax class specifier, ... creates an instance of a Scenic class.

The Scenic class Point provides the basic position properties in the first table below; its subclass OrientedPoint adds the orientation properties in the second table. Finally, the class Object, which represents physical objects and is the default superclass of user-defined Scenic classes, adds the properties in the third table. See the Objects and Classes Reference for details.

Property	Default	Meaning
position 1	(0, 0)	position in global coordinates
viewDistance	50	distance for the ‘can see’ operator
mutationScale	0	overall scale of mutations
positionStdDev	1	mutation standard deviation for position

Properties added by OrientedPoint:

Property	Default	Meaning
heading 1	0	heading in global coordinates
viewAngle	360 degrees	angle for the ‘can see’ operator
headingStdDev	5 degrees	mutation standard deviation for heading

Properties added by Object:

Property	Default	Meaning
width	1	width of bounding box (X axis)
length	1	length of bounding box (Y axis)
speed 1	0	initial speed (later, instantaneous speed)
velocity 1	from speed	initial velocity (later, instantaneous velocity)
angularSpeed 1	0	angular speed (change in heading/time)
behavior	None	dynamic behavior, if any
allowCollisions	False	whether collisions are allowed
requireVisible	True	whether object must be visible from ego
regionContainedIn	workspace	Region the object must lie within
cameraOffset	(0, 0)	position of camera for ‘can see’

1(1,2,3,4,5)

These are dynamic properties, updated automatically every time step during dynamic simulations.

Specifiers

The with property value specifier can specify any property, including new properties not built into Scenic. Additional specifiers for the position and heading properties are listed below.

Illustration of the beyond, behind, and offset by specifiers. Each OrientedPoint (e.g. P) is shown as a bold arrow.

Specifier for position	Meaning
at vector	Positions the object at the given global coordinates
offset by vector	Positions the object at the given coordinates in the local coordinate system of ego (which must already be defined)
offset along direction by vector	Positions the object at the given coordinates, in a local coordinate system centered at ego and oriented along the given direction
(left | right) of vector [by scalar]	Positions the object further to the left/right by the given scalar distance
(ahead of | behind) vector [by scalar]	As above, except placing the object ahead of or behind the given position
beyond vector by vector [from vector]	Positions the object at coordinates given by the second vector, centered at the first vector and oriented along the line of sight from the third vector/ego
visible [from (Point | OrientedPoint)]	Positions the object uniformly at random in the visible region of the ego, or of the given Point/OrientedPoint if given
not visible [from (Point | OrientedPoint)]	Positions the object uniformly at random in the non-visible region of the ego, or of the given Point/OrientedPoint if given

Specifier for position and optionally heading	Meaning
(in | on) region	Positions the object uniformly at random in the given Region
(left | right) of (OrientedPoint | Object) [by scalar]	Positions the object to the left/right of the given OrientedPoint, depending on the object’s width
(ahead of | behind) (OrientedPoint | Object) [by scalar]	As above, except positioning the object ahead of or behind the given OrientedPoint, thereby depending on length
following vectorField [from vector] for scalar	Position by following the given vector field for the given distance starting from ego or the given vector

Specifier for heading	Meaning
facing heading	Orients the object along the given heading in global coordinates
facing vectorField	Orients the object along the given vector field at the object’s position
facing (toward | away from) vector	Orients the object toward/away from the given position (thereby depending on the object’s position)
apparently facing heading [from vector]	Orients the object so that it has the given heading with respect to the line of sight from ego (or the given vector)

Operators

Illustration of several operators. Each OrientedPoint (e.g. P) is shown as a bold arrow.

Scalar Operators	Meaning
relative heading of heading [from heading]	The relative heading of the given heading with respect to ego (or the from heading)
apparent heading of OrientedPoint [from vector]	The apparent heading of the OrientedPoint, with respect to the line of sight from ego (or the given vector)
distance [from vector] to vector	The distance to the given position from ego (or the from vector)
angle [from vector] to vector	The heading to the given position from ego (or the from vector)

Boolean Operators	Meaning
(Point | OrientedPoint) can see (vector | Object)	Whether or not a position or Object is visible from a Point or OrientedPoint.
(vector | Object) in region	Whether a position or Object lies in the region

Heading Operators	Meaning
scalar deg	The given heading, interpreted as being in degrees
vectorField at vector	The heading specified by the vector field at the given position
direction relative to direction	The first direction, interpreted as an offset relative to the second direction

Vector Operators	Meaning
vector (relative to | offset by) vector	The first vector, interpreted as an offset relative to the second vector (or vice versa)
vector offset along direction by vector	The second vector, interpreted in a local coordinate system centered at the first vector and oriented along the given direction

Region Operators	Meaning
visible region	The part of the given region visible from ego
not visible region	The part of the given region not visible from ego

OrientedPoint Operators	Meaning
vector relative to OrientedPoint	The given vector, interpreted in the local coordinate system of the OrientedPoint
OrientedPoint offset by vector	Equivalent to vector relative to OrientedPoint above
(front | back | left | right) of Object	The midpoint of the corresponding edge of the bounding box of the Object, oriented along its heading
(front | back) (left | right) of Object	The corresponding corner of the Object’s bounding box, also oriented along its heading

Built in Functions

Function	Description
Misc Python functions	Various Python functions including min, max, sin, cos, etc.
filter	Filter a possibly-random list (allowing limited randomized control flow).
resample	Sample a new value from a distribution.
localPath	Convert a relative path to an absolute path, based on the current directory.
verbosePrint	Like print, but silent at low-enough verbosity levels.
simulation	Get the the current simulation object.