Start with proven experience, top python development companies not the size of the portfolio. Ask for two or three projects that match your technology stack, and then ask which engineers actually built it. A solid partner is happy to connect you with the engineers. Vague answers at this stage almost always mean you are talking to a reseller.
The paperwork needs more attention than the sales deck. Three sections matter more than the rest: intellectual property assignment, the NDA, and termination and handover. Every artifact must transfer to you as it is paid for, along with designs, scripts and infrastructure configuration. Be careful with any clause that keeps reusable components outside the transfer, because it is usually exactly the piece that locks you in.
Find out how the estimate was built. A credible estimate arrives with a written set of assumptions, a breakdown per feature and an explicit range. A fixed-price contract only makes sense when the specification is complete; when the scope is still moving the vendor prices the risk in and you pay for uncertainty either way. A time-and-materials model puts the risk on your side, so it requires a sprint cadence, demos and a budget cap.
Process matters as much as team size. Ask how a new requirement enters the plan, livewire vs vue comparison who defines done and how quality assurance works. A mature team will be able to show you a working build every one or two weeks. Clear, written acceptance criteria remain the only reliable protection against an argument at delivery time.
Before signing, plan for the handover at the start rather than at the end. Ask that the code repository lives on infrastructure you own from the beginning, and that documentation is written as you go rather than left to the end. A partner who is comfortable with this says yes immediately; resistance at this point reveals quite a lot.