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Commit 6091932

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Merge pull request #132 from ca-nguyen/chain-choice-states
Make Choice states chainable
2 parents 4f90ba3 + aa82ca5 commit 6091932

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2 files changed

+42
-8
lines changed

2 files changed

+42
-8
lines changed

‎src/stepfunctions/steps/states.py

Lines changed: 11 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -218,9 +218,18 @@ def next(self, next_step):
218218
Returns:
219219
State or Chain: Next state or chain that will be transitioned to.
220220
"""
221-
if self.type in ('Choice', 'Succeed', 'Fail'):
221+
if self.type in ('Succeed', 'Fail'):
222222
raise ValueError('Unexpected State instance `{step}`, State type `{state_type}` does not support method `next`.'.format(step=next_step, state_type=self.type))
223223

224+
# By design, Choice states do not have the Next field. When used in a chain, the subsequent step becomes the
225+
# default choice that executes if none of the specified rules match.
226+
# See language spec for more info: https://states-language.net/spec.html#choice-state
227+
if self.type is 'Choice':
228+
if self.default is not None:
229+
logger.warning(f'Chaining Choice state: Overwriting {self.state_id}\'s current default_choice ({self.default.state_id}) with {next_step.state_id}')
230+
self.default_choice(next_step)
231+
return self.default
232+
224233
self.next_step = next_step
225234
return self.next_step
226235

@@ -402,7 +411,7 @@ def allowed_fields(self):
402411
class Choice(State):
403412

404413
"""
405-
Choice state adds branching logic to a state machine. The state holds a list of *rule* and *next_step* pairs. The interpreter attempts pattern-matches against the rules in list order and transitions to the state or chain specified in the *next_step* field on the first *rule* where there is an exact match between the input value and a member of the comparison-operator array.
414+
Choice state adds branching logic to a state machine. The state holds a list of *rule* and *next_step* pairs. The interpreter attempts pattern-matches against the rules in list order and transitions to the state or chain specified in the *next_step* field on the first *rule* where there is an exact match between the input value and a member of the comparison-operator array. When used in a chain, the subsequent step becomes the default choice that executes if none of the specified rules match.
406415
"""
407416

408417
def __init__(self, state_id, **kwargs):

‎tests/unit/test_steps.py

Lines changed: 31 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -12,6 +12,7 @@
1212
# permissions and limitations under the License.
1313
from __future__ import absolute_import
1414

15+
import logging
1516
import pytest
1617

1718
from stepfunctions.exceptions import DuplicateStatesInChain
@@ -346,12 +347,6 @@ def test_append_states_after_terminal_state_will_fail():
346347
chain.append(Succeed('Succeed'))
347348
chain.append(Pass('Pass2'))
348349

349-
with pytest.raises(ValueError):
350-
chain = Chain()
351-
chain.append(Pass('Pass'))
352-
chain.append(Choice('Choice'))
353-
chain.append(Pass('Pass2'))
354-
355350

356351
def test_chaining_steps():
357352
s1 = Pass('Step - One')
@@ -391,6 +386,36 @@ def test_chaining_steps():
391386
assert s2.next_step == s3
392387

393388

389+
def test_chaining_choice_sets_default_field():
390+
s1_pass = Pass('Step - One')
391+
s2_choice = Choice('Step - Two')
392+
s3_pass = Pass('Step - Three')
393+
394+
chain1 = Chain([s1_pass, s2_choice, s3_pass])
395+
assert chain1.steps == [s1_pass, s2_choice, s3_pass]
396+
assert s1_pass.next_step == s2_choice
397+
assert s2_choice.default == s3_pass
398+
assert s2_choice.next_step is None # Choice steps do not have next_step
399+
assert s3_pass.next_step is None
400+
401+
402+
def test_chaining_choice_with_existing_default_overrides_value(caplog):
403+
s1_pass = Pass('Step - One')
404+
s2_choice = Choice('Step - Two')
405+
s3_pass = Pass('Step - Three')
406+
407+
s2_choice.default_choice(s3_pass)
408+
409+
# Chain s2_choice when default_choice is already set will trigger Warning message
410+
with caplog.at_level(logging.WARNING):
411+
Chain([s2_choice, s1_pass])
412+
expected_warning = f'Chaining Choice state: Overwriting {s2_choice.state_id}\'s current default_choice ({s3_pass.state_id}) with {s1_pass.state_id}'
413+
assert expected_warning in caplog.text
414+
assert 'WARNING' in caplog.text
415+
assert s2_choice.default == s1_pass
416+
assert s2_choice.next_step is None # Choice steps do not have next_step
417+
418+
394419
def test_catch_fail_for_unsupported_state():
395420
s1 = Pass('Step - One')
396421

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