PostWhen building a FastAPI application, I pay close attention to how data is validated before it reaches the business logic…

When building a FastAPI application, I pay close attention to how data is validated before it reaches the business logic. That is where Pydantic becomes particularly useful. Pydantic lets us define the structure and rules for the data our application accepts, instead of scattering validation checks throughout the codebase. I use it to define and enforce the data contract at the application boundary. For example: 𝑐𝑙𝑎𝑠𝑠 𝑃𝑟𝑜𝑐𝑒𝑠𝑠𝑖𝑛𝑔𝐶𝑜𝑛𝑓𝑖𝑔(𝐵𝑎𝑠𝑒𝑀𝑜𝑑𝑒𝑙): 𝑐ℎ𝑢𝑛𝑘_𝑠𝑖𝑧𝑒: 𝑖𝑛𝑡 = 𝐹𝑖𝑒𝑙𝑑(𝑔𝑡=0) 𝑜𝑣𝑒𝑟𝑙𝑎𝑝: 𝑖𝑛𝑡 = 𝐹𝑖𝑒𝑙𝑑(𝑔𝑒=0) @𝑚𝑜𝑑𝑒𝑙_𝑣𝑎𝑙𝑖𝑑𝑎𝑡𝑜𝑟(𝑚𝑜𝑑𝑒="𝑎𝑓𝑡𝑒𝑟") 𝑑𝑒𝑓 𝑣𝑎𝑙𝑖𝑑𝑎𝑡𝑒_𝑐𝑜𝑛𝑓𝑖𝑔(𝑠𝑒𝑙𝑓): 𝑖𝑓 𝑠𝑒𝑙𝑓.𝑜𝑣𝑒𝑟𝑙𝑎𝑝 >= 𝑠𝑒𝑙𝑓.𝑐ℎ𝑢𝑛𝑘_𝑠𝑖𝑧𝑒: 𝑟𝑎𝑖𝑠𝑒 𝑉𝑎𝑙𝑢𝑒𝐸𝑟𝑟𝑜𝑟("𝑜𝑣𝑒𝑟𝑙𝑎𝑝 𝑚𝑢𝑠𝑡 𝑏𝑒 𝑠𝑚𝑎𝑙𝑙𝑒𝑟 𝑡ℎ𝑎𝑛 𝑐ℎ𝑢𝑛𝑘_𝑠𝑖𝑧𝑒") 𝑟𝑒𝑡𝑢𝑟𝑛 𝑠𝑒𝑙𝑓 Both fields can be individually valid, while their combination is not. That is the difference between: Field validation → Is this value valid? Model validation → Is this combination valid? With Pydantic, 𝙁𝙞𝙚𝙡𝙙(), 𝙛𝙞𝙚𝙡𝙙_𝙫𝙖𝙡𝙞𝙙𝙖𝙩𝙤𝙧(), 𝙖𝙣𝙙 𝙢𝙤𝙙𝙚𝙡_𝙫𝙖𝙡𝙞𝙙𝙖𝙩𝙤𝙧() let us keep these rules close to the schema. The result is a cleaner boundary: 𝙍𝙚𝙦𝙪𝙚𝙨𝙩 → 𝙑𝙖𝙡𝙞𝙙𝙖𝙩𝙞𝙤𝙣 → 𝘽𝙪𝙨𝙞𝙣𝙚𝙨𝙨 𝙇𝙤𝙜𝙞𝙘 → 𝘿𝙖𝙩𝙖𝙗𝙖𝙨𝙚 / 𝘼𝙄 For production AI applications, this matters. Document metadata, processing parameters, search filters, and structured AI outputs all need predictable contracts. Good schemas do more than describe data. They protect the rest of the system. You can explore more: https://www.youtube.com/playlist?list=PLQNCas8_eikM #Python #Pydantic #PydanticV2 #FastAPI #BackendEngineering #AIEngineering