Future F-strings

1.2.0 · deprecated · verified Mon Apr 13

Future F-strings is a library that backported f-string (formatted string literal) syntax to Python versions prior to 3.6. It enables developers to use f-strings in Python 2.7, and Python 3.3-3.5 environments by transforming the source code. The current version is 1.2.0, but the library is now deprecated as its target Python versions have reached end-of-life and MicroPython, a common target for its `rewrite` functionality, now supports f-strings natively.

Warnings

deprecated The `future-fstrings` library is officially deprecated by its maintainer. Python 2 and Python 3.5 (the target versions for runtime f-string support) have reached their end-of-life. Furthermore, MicroPython, a common target for its source transformation capabilities, now includes native f-string support. The library is no longer needed for its original purpose.
Fix: Upgrade to Python 3.6+ or use native f-string support in MicroPython. If targeting very old Python versions is unavoidable, understand that the library is unmaintained.
gotcha The automatic codec registration via a `.pth` file may not work in certain deployment environments (e.g., AWS Lambda, or when `PYTHONPATH` is used instead of truly installed packages). In such cases, the `future_fstrings.register()` function must be called explicitly before importing any modules that use f-strings.
Fix: Manually register the codec by adding `import future_fstrings; future_fstrings.register()` at the entry point of your application before any f-string-using modules are imported.
gotcha When using the encoding cookie, the entire file is decoded and rewritten at once. While generally robust, complex or very large files *could* theoretically encounter performance issues during the initial load due to this on-the-fly source manipulation.
Fix: For very large files or performance-critical startup paths in Python < 3.6, consider pre-transforming the code using `future-fstrings-show` rather than relying on the runtime codec.

Install

pip install future-fstrings Standard installation for Python < 3.6
pip install future-fstrings[rewrite] Installation for CLI source transformation (Python >= 3.6)

Imports

Encoding Cookie
```
# -*- coding: future_fstrings -*-
# Your f-string code here
```
The primary mechanism for enabling f-strings at runtime in older Python versions is an encoding cookie at the top of the file, not a direct Python import statement. However, `future_fstrings.register()` can be used in specific scenarios.
future_fstrings.register()
```
import future_fstrings
future_fstrings.register()
```
Used for manual registration of the codec in environments where normal .pth file site registration does not occur, such as AWS Lambda or when using PYTHONPATH.

Quickstart

This quickstart demonstrates two ways to use `future-fstrings`. The first shows how to enable f-strings at runtime in a Python < 3.6 environment using the special encoding cookie at the top of the file. The second explains how to use the `future-fstrings-show` command-line utility to transform the source code into Python 2/3.5 compatible format calls, which is useful for distribution to environments without `future-fstrings` installed.

# -*- coding: future_fstrings -*-

import os

name = os.environ.get('USER_NAME', 'world')
message = f'Hello {name}!'
print(message)

# To demonstrate the CLI transformation (requires future-fstrings[rewrite])
# Save the above code as 'my_app.py'
# Run in your shell:
# future-fstrings-show my_app.py
# This will output the Python2/3.5 compatible code (e.g., using .format())

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