flynt - string formatting converter
This is a beta release. Do NOT use on uncommitted code!
flynt is a command line tool to automatically convert a project's Python code from old "%-formatted" and .format(...) strings into Python 3.6+'s "f-strings".
F-Strings:
Not only are they more readable, more concise, and less prone to error than other ways of formatting, but they are also faster!
Installation
pip install flynt. It requires Python version 3.6+.
Usage
To run: flynt {source_file_or_directory}
- Given a single file, it will 'f-stringify' it: replace all applicable string formatting in this file (file will be modified).
- Given a folder, it will search the folder recursively and f-stringify all the .py files it finds. It skips some hard-coded folder names:
blacklist = {'.tox', 'venv', 'site-packages', '.eggs'}.
It turns the code it runs on into Python 3.6+, since 3.6 is when "f-strings" were introduced.
Command line options
usage: flynt [-h] [--verbose | --quiet]
[--no_multiline | --line_length LINE_LENGTH]
src
positional arguments:
src source file or directory
optional arguments:
-h, --help show this help message and exit
--verbose run with verbose output
--quiet run without output
--no_multiline convert only single line expressions
--line_length LINE_LENGTH
for expressions spanning multiple lines, convert only
if the resulting single line will fit into the line
length limit. Default value is 79 characters.
--upgrade run pyupgrade on .py files
Sample output of a successful run:
38f9d3a65222:~ ikkamens$ git clone https://github.com/pallets/flask.git
Cloning into 'flask'...
...
Resolving deltas: 100% (12203/12203), done.
38f9d3a65222:~ ikkamens$ flynt flask
Flynt run has finished. Stats:
Execution time: 0.623s
Files modified: 18
Expressions transformed: 43
Character count reduction: 241 (0.04%)
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Please run your tests before commiting. Report bugs as github issues at: https://github.com/ikamensh/flynt
Thank you for using flynt! Fstringify more projects and recommend it to your colleagues!
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38f9d3a65222:~ ikkamens$
About
Read up on f-strings here:
After obsessively refactoring a project at work, and not even covering 50% of f-string candidates, I realized there was some place for automation. Also it was very interesting to work with ast module.
Dangers of conversion
It is not guaranteed that formatted strings will be exactly the same as before conversion.
'%s' % var is converted to f'{var}'. There is a case when this will behave different from the original - if var is a tuple of one element. In this case, %s displays the element, and f-string displays the tuple. Example:
foo = (1,)
print('%s' % foo) # prints '1'
print(f'{foo}') # prints '(1,)'
Furthermore, some arguments cause formatting of strings to throw exceptions, e.g. print('%d' % 'bla'). While most cases are covered by taking the formatting specifiers to the f-strings format, the precise exception behaviour might differ as well.