python web framework large project
Yes.  An ORM is essential for mapping SQL stuff to objects.  
You have three choices.

Use someone else's ORM
Roll your own.
Try to execute low-level SQL queries and pick out the fields they want from the result set.  This is -- actually -- a kind of ORM with the mappings scattered throughout the applications.  It may be fast to execute and appear easy to develop, but it is a maintenance nightmare.

If you're designing the tables first, any ORM will be painful.  For example, "composite primary key" is generally a bad idea, and with an ORM it's almost always a bad idea.  You'll need to have a surrogate primary key.  Then you can have all the composite keys with indexes you want.  They just won't be "primary".
If you design the objects first, then work out tables that will implement the objects, the ORM will be pleasant, simple and will run quickly, also.
I need your advices to choose a Python Web Framework for developing a large project:
Database (Postgresql)will have at least 500 tables, most of them with a composite primary
key, lots of constraints, indexes & queries. About 1,500 views for starting. The project belongs to the financial area. Alwasy new requirements are coming.
Will a ORM be helpful?