python: dynamic type casting - unicode object to python object conversion

Question

This is a problem I am facing for a data pipeline project. I have 2 data sources. One contains all the user data, the other contains meta data of what all columns we have to process from the user data to output.

So python is good with dynamic type casting, like if I say

a = float
b = "25.123"
c = a(b)
print(c)
>> 25.123

This is what I want to do, I want to dynamically type cast values in order to process them correctly. The type is retrieved from the meta-data data source. The problem is when I do a django model query on the meta-data, I get unicode objects.

a = model.objects.filter(id = 'id') # get the type variable from the meta-data
a = a[0]['type']
print(a)
>> u'float'
a("123.123")
>> TypeError: 'unicode' object is not callable

How do I convert this u'float' to float ? Does this approach have any better alternatives ? I checked out this, but it does not work

Open to all suggestions

Answer 1

In your first example where a = float, a is a built in function, but in your second example, a = u"float", a is an unicode string. If you want complete "dynamicness" of the built in type to convert to without the need of creating a mapping, you can do this:

# for Python 2
a = u"float"
b = "123.123"
import __builtin__
print getattr(__builtin__, a.decode())(b) 
# 123.123

# for Python 3+
a = u"float"
b = "123.123"
import builtins
print(getattr(builtins, a)(b))
# 123.123

---

I would advise you not to use eval() (as suggested by another answer) because it could lead to major security risks. This is why I used the __builtin__/builtins module and getattr() to retrieve the float(...) function.

---

You may also create a mapping (ie. a dict), to map the unicode string to its corresponding function (suggested by this comment):

# both Python 2 and 3
a = u"float"
b = "123.123"
mapping = {u"float": float, u"int": int, u"str": str, u"list": list}
print(mapping[a](b))
# 123.123

Using a mapping is the safest way to go, but it limits your "dynamicness" to only the types listed in the mapping.