Behavior-driven development (BDD) specifies software through expected behavior, then uses those specifications as acceptance criteria. Its scenarios are written collaboratively by business, QA, and development stakeholders, helping turn user requirements into checks that confirm the application meets expectations. section 2.4.4 section 7.3.1
A BDD scenario describes a feature in human-readable Given/When/Then language. Given establishes the starting state, When performs an action, and Then states the expected result. These tests can run from an IDE, but differ from unit tests in how they are designed and the level at which they execute. section 2.4.4 section 4.3.1
Build a BDD test by separating the readable specification from its Python support:
- Create a feature file containing a feature and one or more scenarios in Given/When/Then form. section 4.3.1
- Load the feature scenarios and implement step definitions that connect each phrase to setup, API calls, or assertions. section 4.3.1
- Share state between steps with fixtures when a later assertion needs an API response or processor instance. section 4.3.1 section 7.3.1
- Expand the grammar as new behavior is needed; existing flexible steps can support a new scenario without new code. section 8.6
Checking an API response
A files-endpoint scenario starts with a clean database, posts one or more ACH files, requests the file list, and verifies names, credit and debit totals, and the absence of exceptions. Python step definitions perform the requests, retain the response, and assert the stated conditions. section 7.3.1
Listing 7.11 BDD-style scenarios
Feature: The /files endpoint
Test the functionality of the /files endpoint
Scenario: I want to get a list that contains a single file
Given that I have a clean database
And that I have posted the file "sample.ach"
When I request a list of files
Then I should have a single file named "sample.ach"
And it should have a total credit amount of "1146114.80"
And it should have a total debit amount of "78.25"
And there should be no exceptions
Scenario: I want to get a list of files
Given that I have a clean database
And that I have posted the file "sample.ach"
And that I have posted the file "sample1.ach"
When I request a list of files
Then I should have a file that includes the file "sample.ach"
And I should have a file that includes the file "sample1.ach"
And there should be no exceptions
This scenario shows how stakeholder-readable setup, API actions, and business-facing assertions become an executable test. section 7.3.1
BDD can also guide design before implementation. A transaction-search scenario prompts decisions about searchable fields, whether criteria combine with AND or OR, and whether the UI or server interprets a single search input. The scenario then defines a concrete expected result for the chosen behavior. section 9.2.1
Use BDD-style syntax to create test data as well as verify behavior. File-creation scenarios can describe batch counts, debit or credit entries, amounts, names, and expected file contents, enabling short cycles that expose problems across generation, parsing, APIs, and the UI. Such generated files may not enforce every business rule unless that behavior is explicitly added. section 7.6
Treat scenarios as first-class work alongside production code. As requirements grow, add cases for record counts, batches, exceptions, and new endpoint behavior; run broader or slower tests through CI/CD when component dependencies make them unsuitable for a rapid development suite. section 7.3.1 section 4.3.1
