"WIP: Initial PSBT (BIP-174) data structures"

[JAGADISHSUNILPEDNEKAR]

WIP: Initial PSBT (BIP-174) data structures

This PR adds the initial data structures for BIP-174 PSBT implementation. I've focused on creating a minimal yet complete foundation that follows the specification while maintaining clean code practices.

What's included:

• Basic PSBT class structure with core fields: Implemented the PSBT class with proper version number handling and container management that follows BIP-174 specifications
• Input and Output container classes: Created dedicated classes that encapsulate the data structure requirements for PSBT inputs/outputs
• PSBT type constants as defined in BIP-174: Added constants for all key types to ensure type-safety and readability throughout the implementation
• Simple example demonstrating creation of a PSBT object: Added an example that shows how to construct a PSBT from an unsigned transaction
• Basic tests to verify the implementation: Created focused tests that verify the core functionality

Design decisions:

• I chose to implement PSBT with composition over inheritance to maintain clean separation between transaction elements and their PSBT metadata
• The core PSBT class maintains references to inputs/outputs from the original transaction to ensure data consistency
• Implemented careful property access patterns to prevent direct modification of critical fields
• Fixed the output index access in the example script by using the stored value rather than trying to access a non-existent attribute on the TxInput object

Current limitations:

• Serialization/deserialization not yet implemented (planned for next PR)
• Not yet exposed in __init__.py to avoid breaking changes
• Role-specific functionality (Creator, Updater, etc.) to come in future PRs

Running tests and examples:

The test can be run with: cd tests && python test_psbt.py
The example can be run with: cd examples && python psbt_create_basic.py

This is the first step toward full PSBT implementation. Next PRs will build on this foundation with serialization and role-specific functionality.

[JAGADISHSUNILPEDNEKAR]

All the testcases are passing

[JAGADISHSUNILPEDNEKAR]

Hi @karask as I have implemented the initial data structures for BIP-174 support including the core PSBT class, input/output containers and the key constants , also added the basic example and a test case to validate the functionality while laying down a clean modular serialisation and role based enhancements

In addressing the output index issue specifically, I decided to use the stored value rather than trying to access a non-existent attribute on the TxInput object, which provides a cleaner solution without requiring changes to the transaction classes.

Further I intend to implement from_bytes and to_bytes methods for PSBT serialization/deserialization as per BIP-174.

Can you please let me know what do you think about this?
Also let me know if you need any changes in the current implementation I would be happy to implement that