Question

I have an SQL template where I need to substitute random UUIDs.

from string import Template
import uuid

def gen_uuid():
    return str(uuid.uuid4())

s = """
insert ... values('foo', ${uuid});
insert ... values('bar', ${uuid});
insert ... values('baz', ${uuid});
"""

mappings = {
    "uuid": gen_uuid  # how??
}

template = Template(s)
print template.safe_substitute(mappings)

How can I bind different UUIDs values to the same template key?

Update

Ok.. I ended up overriding getitem. Not ideal but works..

class UUIDDict(dict):
    def __init__(self, *args, **kwargs):
        self.update(*args, **kwargs)

    def __getitem__(self, key):
        if key == 'uuid':
            return str(uuid.uuid4())

        return super(UUIDDict, self).__getitem__(key)

Answer 1

string.Template() isn’t designed to substitute multiple values for a single key. Other templating systems may support this, but string.Template is meant to be dirt simple.

That being said, it isn’t hard to do what you want in a loop and then combine the results using str.join:

>>> from string import Template
>>> import uuid
>>> def gen_uuid():
        return str(uuid.uuid4())

>>> s = Template("insert ... values('$name', ${uuid});")
>>> t = [s.substitute(name=name, uuid=gen_uuid()) for name in ('foo', 'bar', 'baz')]
>>> print '\n'.join(t)
insert ... values('foo', ee1b1b21-c022-4de5-8f7c-0a4d0554ae49);
insert ... values('bar', 0b96872f-ab0e-48cf-a025-f997f7976a0e);
insert ... values('baz', 25165bb6-8b7b-4c87-9ce1-ca274fc51bc8);

As suggested by the OP, a custom dictionary can be created that will build a new uuid() on each lookup:

>>> class UUID_Dict(dict):
        def __missing__(self, key):
            if key == 'uuid':
                 return str(uuid.uuid4())
            raise KeyError(key)


>>> s = Template("""
insert ... values('foo', ${uuid});
insert ... values('bar', ${uuid});
insert ... values('baz', ${uuid});
""")
>>> print s.substitute(UUID_Dict())

insert ... values('foo', eabc65b6-1294-43b7-9506-61a4e324e0f2);
insert ... values('bar', 93e4f0b7-7fa1-4e88-9696-e361da31358f);
insert ... values('baz', 503d34a3-17a4-4513-8ce0-b5efb70c94cc);