TagStr

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.

Installation

$ pip install tagstr

At a Glance

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.

Examples

While the example above isn't especially useful,