Converts all Python 3.6 fstrings f"insert {var:.2f} here" to Python 3.5 compatible string.format "insert {:.2f} here".format(var)

3.6_fstring-3.5_strformat.py

##############################################################################
##                        3.6_fstring-3.5_strformat.py                      ##
##                             Matt Moran (2018)                            ##
##############################################################################
## Usage: python 3.6_fstring-3.5_strformat.py file_to_process.py
##
## Converts all 3.6 fstrings f"insert {var:.2f} here" to 3.5 compatible
##   "insert {:.2f} here".format(var)
##
## Works with:
##   single and triple quoted strings               f"""{var}"""
##   format specifiers                              f'{var:.2f}'
##
## DOES NOT WORK WITH:
##   literal braces                                 f"{{this is a string}}"
##   strings that open and close multiple times     f"{x}" "continues" "here"
##   

#!/usr/bin/env python3
import re
import sys

# get filename passed from command line
filename = sys.argv[1]

# strings that open with  ' and " triple quoted strings
fstring_patterns = [r'f"(.*)?"', r"f'(.*)?'", "f'''(.*)?'''", 'f"""(.*)?"""']
brace_pattern = re.compile(r'{([^:{]+)(:[^}]+)?}')

# open and read input file
with open(filename, "r") as f:
    fread = f.read();

# loop through each fstring regex pattern
for fstring_pattern in fstring_patterns:
    # loop through the file finding matches
    for pattern_match in re.finditer(fstring_pattern, fread):
        string = pattern_match.group(0)[1:] # cut starting f off the string
        variables = []
        # get all the brace expressions in each match
        for n in brace_pattern.finditer(string):
            # extract format specifier
            format = n.group(2) if n.group(2) else ""
            # remove group 1, the name of the variable
            string = string.replace(n.group(0), "{" + format + "}")
            # save the variable to a list to pass to format
            variables.append(n.group(1))
        
        # make the string, with the .format bit
        string += ".format(" + ", ".join(variables) + ")"
        # replace the old fstring with the new string.format
        fread = fread.replace(pattern_match.group(0), string)
        
# write the output
filename = filename.rsplit(".", 1)
with open(filename[0] + "_out." + filename[1], "w") as f:
    f.write(fread)