Lexical cast from string to type

Question

Recently, I was trying to store and read information from files in Python, and came across a slight problem: I wanted to read type information from text files. Type casting

Answer 1

Why not just use a look-up table?

known_types = {
    'int': int,
    'float': float,
    'str': str
    # etc
}

var_type = known_types['int']

Answer 2

I like using locate, which works on built-in types:

>>> from pydoc import locate
>>> locate('int')
<type 'int'>
>>> t = locate('int')
>>> t('1')
1

...as well as anything it can find in the path:

>>> locate('datetime.date')
<type 'datetime.date'>
>>> d = locate('datetime.date')
>>> d(2015, 4, 23)
datetime.date(2015, 4, 23)

...including your custom types:

>>> locate('mypackage.model.base.BaseModel')
<class 'mypackage.model.base.BaseModel'>
>>> m = locate('mypackage.model.base.BaseModel')
>>> m()
<mypackage.model.base.BaseModel object at 0x1099f6c10>

Answer 3

You're a bit confused on what you're trying to do. Types, also known as classes, are objects, like everything else in python. When you write int in your programs, you're referencing a global variable called int which happens to be a class. What you're trying to do is not "cast string to type", it's accessing builtin variables by name.

Once you understand that, the solution is easy to see:

def get_builtin(name):
    return getattr(__builtins__, name)

---

If you really wanted to turn a type name into a type object, here's how you'd do it. I use deque to do a breadth-first tree traversal without recursion.

def gettype(name):
    from collections import deque
    # q is short for "queue", here
    q = deque([object])
    while q:
        t = q.popleft()
        if t.__name__ == name:
            return t
        else:
            print 'not', t

        try:
            # Keep looking!
            q.extend(t.__subclasses__())
        except TypeError:
            # type.__subclasses__ needs an argument, for whatever reason.
            if t is type:
                continue
            else:
                raise
    else:
        raise ValueError('No such type: %r' % name)

Answer 4

Perhaps this is what you want, it looks into builtin types only:

def gettype(name):
    t = getattr(__builtins__, name)
    if isinstance(t, type):
        return t
    raise ValueError(name)