python：使用字典键的一部分作为字典值的变量

Question

I have dictionary that looks like this:

mapper = {
    "bigint(20)": "= Column(BigInteger)",
    "int(11)": "= Column(Integer)",
    "`": "",
    "varchar(255)": "= Column(String)",
}

I am mapping sql datatype naming into sqlalchemy-like to be replaced into a text file. So far I am just mapping int to integer, varchar to string and so on. However, my idea is to use part of the key as variable to be passed to its relative value, ie avoiding to hardcode varchar(255) and use varchar(x) to be passed to its relative value = Column(String()) having = Column(String(x)) .

Something like:

"varchar(x)": "= Column(String{})".format(x),

Do you have any suggestions how to achieve that? thanks

Answer 1

A dict is a fixed set of concrete keys; you cannot do what you want with a dict alone.

However, you can subclass dict to provide a definition of __getitem__ that can do what you want. Something like

import re
class SQLDict(dict):
    def __getitem__(self, key):
        m = re.match('^varchar\((.*)\)$', key)
        if m:
            return super(SQLDict, self).__getitem__('varchar(x)') % (m.group(1),)
        else:
            return super(SQLDIct,self).__getitem__(key)

mapper = SQLDict({
    "bigint(20)": "= Column(BigInteger)",
    "int(11)": "= Column(Integer)",
    "`": "",
    "varchar(x)": "= Column(String(%s))",
})

Trying it out...

>>> print mapper['varchar(15)']
'=Column(String(215))'

It's not perfect; I've hard-coded varchar(x) into the definition of __getitem__ , while you might define mapper to have a key like varchar(y) instead. But this should get you started.