Transitions

Note

While actions form the steps taken in an application, transitions decide which one to do next. They make decisions based on state. You can use them to specify which model to call, whether a conversation is over, or any other decision that needs to be made based on the current state.

Transitions define explicitly how actions are connected and which action is available next for any given state. You can think of them as edges in a graph.

They have three main components: - The from state - The to state - The condition that must be met to move from the from state to the to

Conditions

Conditions have a few APIs, but the most common are the three convenience functions:

from burr.core import when, expr, default
with_transitions(
    ("from", "to", when(foo="bar"),  # will evaluate when the state has the variable "foo" set to the value "bar"
    ("from", "to", expr('epochs>100')) # will evaluate to True when epochs is greater than 100
    ("from", "to", default)  # will always evaluate to True
    ("from", "to") # leaving out a third conditions we allow defaults
)

when() also supports comparison operators via Django-style __ suffixes:

from burr.core import when
with_transitions(
    ("check", "adult", when(age__gte=18)),       # age >= 18
    ("check", "child", when(age__lt=18)),         # age < 18
    ("check", "valid", when(score__gt=0, score__lte=100)),  # 0 < score <= 100
    ("check", "active", when(status__in=["active", "pending"])),  # membership
    ("check", "tagged", when(tags__contains="python")),  # collection contains value
    ("check", "clean", when(status__notin=["banned", "suspended"])),  # not in
    ("check", "changed", when(status__ne="initial")),  # not equal
    ("check", "missing", when(value__is=None)),  # identity check
    ("check", "present", when(value__isnot=None)),  # not-identity check
)

Available operators:

• key=value — exact equality (default, unchanged)

• key__eq=value — explicit equality

• key__ne=value — not equal

• key__gt=value — greater than

• key__gte=value — greater than or equal

• key__lt=value — less than

• key__lte=value — less than or equal

• key__in=[values] — value is in the given collection

• key__notin=[values] — value is not in the given collection

• key__contains=value — collection/string in state contains the value

• key__is=value — identity check (is), useful for None/True/False

• key__isnot=value — negated identity check (is not)

Multiple keyword arguments are ANDed together. For more complex expressions, use expr().

Conditions are evaluated in the order they are specified, and the first one that evaluates to True will be the transition that is selected when determining which action to run next. If no condition evaluates to True, the application execution will stop early.

The ~ operator will invert a condition. For instance:

from burr.core import when, expr
with_transitions(
    ("from", "to", ~when(foo="baz"),  # will evaluate to True when foo != baz
    ("from", "to", ~expr('epochs<=100')) # will evaluate to True when epochs hits 101
)

All keys present in the condition (E.G. foo and epochs above) must be present in the state for the condition to work. It will error otherwise.

Note

The default condition is a special case, and will always evaluate to True. It is useful for defining a “catch-all” transition that will be selected if no other condition is met. If you pass a tuple of length 2 to with_transitions, the default condition will be used.

See the transition docs for more information on the transition API.