Statements Reference

Compound Statements

Class Definition

class <name>[(<superclass>)]:
    [<property>: <value>]*

Defines a Scenic class. If a superclass is not explicitly specified, Object is used (see Objects and Classes Reference). The body of the class defines a set of properties its objects have, together with default values for each property. Properties are inherited from superclasses, and their default values may be overridden in a subclass. Default values may also use the special syntax to refer to one of the other properties of the same object, which is then a dependency of the default value. The order in which to evaluate properties satisfying all dependencies is computed (and cyclic dependencies detected) during Specifier Resolution.

Scenic classes may also define attributes and methods in the same way as Python classes.

Behavior Definition

behavior <name>(<arguments>):
    [precondition: <boolean>]*
    [invariant: <boolean>]*

Defines a dynamic behavior, which can be assigned to a Scenic object by setting its behavior property using the with behavior behavior specifier; this makes the object an agent. See our tutorial on Dynamic Scenarios for examples of how to write behaviors.

Behavior definitions have the same form as function definitions, with an argument list and a body consisting of one or more statements; the body may additionally begin with definitions of preconditions and invariants. Preconditions are checked when a behavior is started, and invariants are checked at every time step of the simulation while the behavior is executing (including time step zero, like preconditions, but not including time spent inside sub-behaviors: this allows sub-behaviors to break and restore invariants before they return).

The body of a behavior executes in parallel with the simulation: in each time step, it must either take specified action(s) or wait and perform no actions. After each take or wait statement, the behavior’s execution is suspended, the simulation advances one step, and the behavior is then resumed. It is thus an error for a behavior to enter an infinite loop which contains no take or wait statements (or do statements invoking a sub-behavior; see below): the behavior will never yield control to the simulator and the simulation will stall.

Behaviors end naturally when their body finishes executing (or if they return): if this happens, the agent performing the behavior will take no actions for the rest of the scenario. Behaviors may also terminate the current scenario, ending it immediately.

Behaviors may invoke sub-behaviors, optionally for a limited time or until a desired condition is met, using do statements. It is also possible to (temporarily) interrupt the execution of a sub-behavior under certain conditions and resume it later, using try-interrupt statements.

Monitor Definition

monitor <name>(<arguments>):

Defines a type of monitor, which can be run in parallel with the simulation like a dynamic behavior. Monitors are not associated with an Object and cannot take actions, but can wait to wait for the next time step (or use terminate or terminate simulation to end the scenario/simulation). A monitor can be instantiated in a scenario with the require monitor statement.

The main purpose of monitors is to evaluate complex temporal properties that are not expressible using the temporal operators available for require LTL formula statements. They can maintain state and use require to enforce requirements depending on that state. For examples of monitors, see our tutorial on Dynamic Scenarios.

Changed in version 3.0: Monitors may take arguments, and must be explicitly instantiated using a require monitor statement.

Modular Scenario Definition

scenario <name>(<arguments>):
    [precondition: <boolean>]*
    [invariant: <boolean>]*
scenario <name>(<arguments>):

Defines a Scenic modular scenario. Scenario definitions, like behavior definitions, may include preconditions and invariants. The body of a scenario consists of two optional parts: a setup block and a compose block. The setup block contains code that runs once when the scenario begins to execute, and is a list of statements like a top-level Scenic program (so it may create objects, define requirements, etc.). The compose block orchestrates the execution of sub-scenarios during a dynamic scenario, and may use do and any of the other statements allowed inside behaviors (except take, which only makes sense for an individual agent). If a modular scenario does not use preconditions, invariants, or sub-scenarios (i.e., it only needs a setup block) it may be written in the second form above, where the entire body of the scenario comprises the setup block.

See also

Our tutorial on Composing Scenarios gives many examples of how to use modular scenarios.

Try-Interrupt Statement

[interrupt when <boolean>:
[except <exception> [as <name>]:

A try-interrupt statement can be placed inside a behavior (or compose block of a modular scenario) to run a series of statements, including invoking sub-behaviors with do, while being able to interrupt at any point if given conditions are met. When a try-interrupt statement is encountered, the statements in the try block are executed. If at any time step one of the interrupt conditions is met, the corresponding interrupt block (its handler) is entered and run. Once the interrupt handler is complete, control is returned to the statement that was being executed under the try block.

If there are multiple interrupt clauses, successive clauses take precedence over those which precede them; furthermore, during execution of an interrupt handler, successive interrupt clauses continue to be checked and can interrupt the handler. Likewise, if try-interrupt statements are nested, the outermost statement takes precedence and can interrupt the inner statement at any time. When one handler interrupts another and then completes, the original handler is resumed (and it may even be interrupted again before control finally returns to the try block).

The try-interrupt statement may conclude with any number of except blocks, which function identically to their Python counterparts (though Scenic does not allow except* blocks).

Simple Statements

The following statements can occur throughout a Scenic program unless otherwise stated.

model name

Select a world model to use for this scenario. The statement model X is equivalent to from X import * except that X can be replaced using the --model command-line option or the model keyword argument to the top-level APIs. When writing simulator-agnostic scenarios, using the model statement is preferred to a simple import since a more specific world model for a particular simulator can then be selected at compile time.

import module

Import a Scenic or Python module. This statement behaves as in Python, but when importing a Scenic module it also imports any objects created and requirements imposed in that module. Scenic also supports the form from module import identifier, ... , which as in Python imports the module plus one or more identifiers from its namespace.

param name = value, …

Defines one or more global parameters of the scenario. These have no semantics in Scenic, simply having their values included as part of the generated Scene, but provide a general-purpose way to encode arbitrary global information.

If multiple param statements define parameters with the same name, the last statement takes precedence, except that Scenic world models imported using the model statement do not override existing values for global parameters. This allows models to define default values for parameters which can be overridden by particular scenarios. Global parameters can also be overridden at the command line using the --param option, or from the top-level API using the params argument to scenic.scenarioFromFile.

To access global parameters within the scenario itself, you can read the corresponding attribute of the globalParameters object. For example, if you declare param weather = 'SUNNY', you could then access this parameter later in the program via If the parameter was not overridden, this would evaluate to 'SUNNY'; if Scenic was run with the command-line option --param weather SNOW, it would evaluate to 'SNOW' instead.

Some simulators provide global parameters whose names are not valid identifiers in Scenic. To support giving values to such parameters without renaming them, Scenic allows the names of global parameters to be quoted strings, as in this example taken from an X-Plane scenario:

param simulation_length = 30
param 'sim/weather/cloud_type[0]' = DiscreteRange(0, 5)
param 'sim/weather/rain_percent' = 0

require boolean

Defines a hard requirement, requiring that the given condition hold in all instantiations of the scenario. This is equivalent to an “observe” statement in other probabilistic programming languages.

require[number] boolean

Defines a soft requirement; like require above but enforced only with the given probability, thereby requiring that the given condition hold with at least that probability (which must be a literal number, not an expression). For example, require[0.75] ego in parking_lot would require that the ego be in the parking lot at least 75% percent of the time.

require LTL formula

Defines a temporal requirement, requiring that the given Linear Temporal Logic formula hold in a dynamic scenario. See Temporal Operators for the list of supported LTL operators.

Note that an expression that does not use any temporal operators is evaluated only in the current time step. So for example:

  • require A and always B will only require that A hold at time step zero, while B must hold at every time step (note that this is the same behavior you would get if you wrote require A and require always B separately);

  • require (always A) implies B requires that if A is true at every time step, then B must be true at time step zero;

  • require always A implies B requires that in every time step when A is true, B must also be true (since B is within the scope of the always operator).

require monitor monitor

Require a condition encoded by a monitor hold during the scenario. See Monitor Definition for how to define types of monitors.

It is legal to create multiple instances of a monitor with varying parameters. For example:

monitor ReachesBefore(obj1, region, obj2):
    reached = False
    while not reached:
        if obj1 in region:
            reached = True
            require obj2 not in region

require monitor ReachesBefore(ego, goal, racecar2)
require monitor ReachesBefore(ego, goal, racecar3)

terminate when boolean

Terminates the scenario when the provided condition becomes true. If this statement is used in a modular scenario which was invoked from another scenario, only the current scenario will end, not the entire simulation.

terminate simulation when boolean

The same as terminate when, except terminates the entire simulation even when used inside a sub-scenario (so there is no difference between the two statements when used at the top level).

terminate after scalar (seconds | steps)

Like terminate when above, but terminates the scenario after the given amount of time. The time limit can be an expression, but must be a non-random value.

mutate identifier, … [by scalar]

Enables mutation of the given list of objects (any Point, OrientedPoint, or Object), with an optional scale factor (default 1). If no objects are specified, mutation applies to every Object already created.

The default mutation system adds Gaussian noise to the position and heading properties, with standard deviations equal to the scale factor times the positionStdDev and headingStdDev properties.


User-defined classes may specify custom mutators to allow mutation to apply to properties other than position and heading. This is done by providing a value for the mutator property, which should be an instance of Mutator. Mutators inherited from superclasses (such as the default position and heading mutators from Point and OrientedPoint) will still be applied unless the new mutator disables them; see Mutator for details.

record [initial | final] value [as name]

Record the value of an expression during each simulation. The value can be recorded at the start of the simulation (initial), at the end of the simulation (final), or at every time step (if neither initial nor final is specified). The recorded values are available in the records dictionary of SimulationResult: its keys are the given names of the records (or synthesized names if not provided), and the corresponding values are either the value of the recorded expression or a tuple giving its value at each time step as appropriate. For debugging, the records can also be printed out using the --show-records command-line option.

Dynamic Statements

The following statements are valid only in dynamic behaviors, monitors, and compose blocks.

take action, …

Takes the action(s) specified and pass control to the simulator until the next time step. Unlike wait, this statement may not be used in monitors or modular scenarios, since these do not take actions.


Take no actions this time step.


Immediately end the scenario. As for terminate when, if this statement is used in a modular scenario which was invoked from another scenario, only the current scenario will end, not the entire simulation. Inside a behavior being run by an agent, the “current scenario” for this purpose is the scenario which created the agent.

terminate simulation

Immediately end the entire simulation.

do behavior/scenario, …

Run one or more sub-behaviors or sub-scenarios in parallel. This statement does not return until all invoked sub-behaviors/scenarios have completed.

do behavior/scenario, … until boolean

As above, except the sub-behaviors/scenarios will terminate when the condition is met.

do behavior/scenario for scalar (seconds | steps)

Run sub-behaviors/scenarios for a set number of simulation seconds/time steps. This statement can return before that time if all the given sub-behaviors/scenarios complete.

do choose behavior/scenario, …

Randomly pick one of the given behaviors/scenarios whose preconditions are satisfied, and run it. If no choices are available, the simulation is rejected.

This statement also allows the more general form do choose { behaviorOrScenario: weight, ... }, giving weights for each choice (which need not add up to 1). Among all choices whose preconditions are satisfied, this picks a choice with probability proportional to its weight.

do shuffle behavior/scenario, …

Like do choose above, except that when the chosen sub-behavior/scenario completes, a different one whose preconditions are satisfied is chosen to run next, and this repeats until all the sub-behaviors/scenarios have run once. If at any point there is no available choice to run (i.e. we have a deadlock), the simulation is rejected.

This statement also allows the more general form do shuffle { behaviorOrScenario: weight, ... }, giving weights for each choice (which need not add up to 1). Each time a new sub-behavior/scenario needs to be selected, this statement finds all choices whose preconditions are satisfied and picks one with probability proportional to its weight.


Used in an interrupt handler to terminate the current try-interrupt statement.

override object specifier, …

Override one or more properties of an object, e.g. its behavior, for the duration of the current scenario. The properties will revert to their previous values when the current scenario terminates. It is illegal to override dynamic properties, since they are set by the simulator each time step and cannot be mutated manually.