python - 字符串模板 - 相同键的不同值

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

The first problem is you're not passing the result of gen_uuid to the template, you're referencing the function object itself. It should be:

mappings = {
    "uuid": gen_uuid()
}

With your current approach the gen_uuid function will only be evaluated once, so the same value will be used for each statement.

Why not do something like this instead?:

names = ['foo', 'bar', 'baz']

for name in names:
    c.execute("INSERT into foo (a, b) VALUES (%s, %s)", (name, gen_uuid()))

Answer 2

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);

Answer 3

Here is a solution that uses a "default dictionary" ( defaultdict ):

from collections import defaultdict
from string import Template
import uuid

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

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

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

The result will be something like the following:

insert ... values('foo', aedd5fb6-40da-45bd-bcf7-c7390ef8f13e);
insert ... values('bar', c7ea3090-2a0f-49aa-ace9-c5ef67b7888b);
insert ... values('baz', bf2d86b7-c13e-4498-8d4a-27e76c2e72ab);

A defaultdict is created with a single argument, a factory function. Every time an unknown key is looked up in a default dictionary, it calls that function, binds that value to the key, and then returns the key. Thus it will assign a new UUID to each variable.

The main disadvantage is that you can't use the single variable ${uuid} ; You will have to define a new variable for each uuid. Then again, this may be better practice in the long run: that way, you can access the particular UUID for each value in the template should you need it again. You could even access the UUIDs direct from the Python code, after you do the template substitution:

>>> for k, v in mappings.items():
...     print k, v
... 
baz_uuid bf2d86b7-c13e-4498-8d4a-27e76c2e72ab
bar_uuid c7ea3090-2a0f-49aa-ace9-c5ef67b7888b
foo_uuid aedd5fb6-40da-45bd-bcf7-c7390ef8f13e