Tagged template literals for Python.

This project seeks to implement an early stage PEP for adding "tag strings", also known as tagged template literals, using import hooks. Tag strings, are a natural extension of f-strings which enable Python developers to create custom string interpolation logic.

$ pip install tagstr

Here's a simple hello world example:

# tagstr: on
name = "world"
print @ f"Hello, {name!r}!"

In the code above, print is knows as a "tag function", @ is the "tag operator", f"Hello, {name!r}!" is the "tag string", and name is one of its interpolated values:

print @ f"Hello, {name!r}!"
# │   │  │         └── interpolated value
# │   │  │
# │   |  └── tag string
# │   │
# │   └── tag operator
# │
# └── tag function

When run, the example above prints:

Hello,  (<function <lambda> at 0x7f918e063380>, 'name', 'r', None) !

This happens because tagstr sees the tagstr: on comment and rewrites it as:

# tagstr: on
name = "world"
print("Hello, ", (lambda: name, "name", "r", None), "!")

Not that the tag function (in this case print) gains access to the original string as well as a tuple with information about the interpolated {name!r} called a "thunk". In particular, this thunk contains a lambda for lazily retrieving that value.

While the example above isn't especially useful,