safe python micro-template

program.py

# see also:
# - https://docs.python.org/3/library/string.html#string.Template
# - https://docs.python.org/3/library/string.html#string.Template.safe_substitute
# - https://docs.python.org/3/library/stdtypes.html#str.format_map

class _identity_format(dict):
    def __missing__(self, key):
        return f'{{{key}}}'

class T:
    def __init__(self, s):
        self._s = s
    def __repr__(self):
        return f'T({self._s!r})'
    def __call__(self, **kwargs):
        s = self._s.format_map(_identity_format(**kwargs))
        return self.__class__(s)

In [2]: s = T('{a} {b} {c}')                                                                                                                          

In [3]: s(a=1)                                                                                                                                        
Out[3]: T('1 {b} {c}')

In [4]: s(a=1)(b=2)                                                                                                                                   
Out[4]: T('1 2 {c}')

In [5]: s(a=1)(b=2)(c=3)                                                                                                                              
Out[5]: T('1 2 3')
    

# another approach, this time inherit from str

class T2(str):
     def __repr__(self):
        return f'{self.__class__.__name__}({self})'
     def format(self, **kwargs): 
         s = super().format_map(_identity_format(**kwargs))
         return self.__class__(s)


In [6]: s = T2('{a} {b} {c}')                                                                                                                        

In [7]: s.format(a=1)                                                                                                                                
Out[7]: T2('1 {b} {c}')

In [8]: s.format(a=1).format(b=2)                                                                                                                    
Out[8]: T2('1 2 {c}')

In [9]: s.format(a=1).format(b=2).format(c=3)                                                                                                        
Out[9]: T2('1 2 3')