python - Lexical cast from string to type

35
votes

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 from string to int or to float is quite efficient, but type casting from string to type seems to be another problem. Naturally, I tried something like this:

var_type = type('int')

However, type isn't used as a cast but as a mechanism to find the type of the variable, which is actually str here.

I found a way to do it with:

var_type = eval('int')

But I generally try to avoid functions/statements like eval or exec where I can. So my question is the following: Is there another pythonic (and more specific) way to cast a string to a type?

4
pythoncasting
Perhaps you should use a different format than text to store your information? Python has many data persistence options available. - Martijn Pieters♦
Adding to the above, you could store your data by pickling it. docs.python.org/library/pickle.html - Hubro
One possibility is to use predefined dictionary with strings as keys and types as values. Although I recommend to refactor the code. Dynamic script loading ( which is basically what you do ) is always a problem. - freakish
@Codemonkey: pickle is listed on the page I link to, first option. :-) - Martijn Pieters♦
@MartijnPieters: I assumed as much, but I was already typing the comment and I'm lazy :-P - Hubro

4 Answers

47
votes

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>
11
votes

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)
7
votes

Why not just use a look-up table?

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

var_type = known_types['int']
4
votes

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)