CODEWITHSUNDEEP
python
ssd
ssd

Reputation: 2442

Declaring a strict data type for a variable in Python?

In python, is there a command (or a directive) that raises warning when a variable is assigned a value that differs from the previously assigned type?

x = int()   # "x" declared as integer
y = float() # "y" declared as float
x = 5       # "x" assigned an integer
y = 2.75    # "y" assigned a float
print(x)    # prints "5"
print(y)    # prints "2.75"
x = y       # !!! "x" is assigned a float; no warning raised !!!
print(x)    # prints 2.75

Upvotes: 8

Views: 23128

Answers (2)

Dobedani
Dobedani

Reputation: 578

Python versions 3.6 and higher support type checking. Check out this article:
How to Use Static Type Checking in Python 3.6
It was written by Adam GeitGey - back in 2017. No need to use a different language!

Upvotes: 7

tdelaney
tdelaney

Reputation: 77407

You can't control assignment of global or local variables, but you can override assignment of class object attributes. Here is a class that uses setattr to enforce type. It has a method to assign static type (use int not int() for example) and can also assign type on first assignment of a variable. It is very strict about type but can be changed to allow inherited types.

class BabySitter(object):

    def __init__(self):
        object.__setattr__(self, "_types", {})

    # if you want static assignment
    def set_type(self, name, _type):
        self._types[name] = _type

    def __setattr__(self, name, value):
        _type = self._types.get(name)
        if _type:
            if type(value) is not _type: # or `if not isinstance(value, _type)`
                raise ValueError(
                    "BabySitter type conflict assigning '{}': was {} is {}".format(
                    name, _type, type(value)))
        # if you want dynamic assignment
        else:
            self._types[name] = type(value)
        object.__setattr__(self, name, value)

var = BabySitter()
var.set_type("x", int)     # static "x" declared as integer
var.set_type("y", float)   # static "y" declared as float
var.z = 123     # dynamic "z" int because of first assignment
var.x = 5       # "x" assigned an integer
var.y = 2.75    # "y" assigned a float
print(var.x)        # prints "5"
print(var.y)        # prints "2.75"
var.x = var.y           # <== exception is raised
print(var.x)        # prints 2.75

Upvotes: 7

Related Questions