Convert string variables into other types

Question

I have a class which hold some variables as strings. e.g.

x = "1"
y = "2.0"
z = "timedelta(seconds=10)"

As you can see above the variables are actually int/float/timedelta-types. I need to return these variables with their real type. (x should be returned as a int, y should be returned as a float, z should be returned as datetime.timedelta)

I was thinking of changing the getattr and try to find out the type before returning the value, but the small test I did, does not seem to work:

from datetime import datetime, timedelta

def test():
    string = "timedelta(seconds=10)"
    x = eval(string)
    print x
    print type(x)
    if type(x) == 'datetime.timedelta':
        print "YES"
    else:
        print "NO"

The output is:

0:00:10
<type 'datetime.timedelta'>
NO

why is the if-case returning false? is there a better way to return these variables with the real type?

score 4 · Accepted Answer · answered Oct 18 '12 at 07:14

4

Because type(x) returns type not string. Instead use:

type(x) == datetime.timedelta

# or, the better Python practice

isinstance(x, datetime.timedelta)

answered Oct 18 '12 at 07:14

Zaur Nasibov

22,280
12
56
83

does not seem to work.Traceback (most recent call last): File "eval.py", line 14, in test() File "eval.py", line 8, in test if isinstance(x, datetime.timedelta): AttributeError: type object 'datetime.datetime' has no attribute 'timedelta' – theAlse Oct 18 '12 at 07:56
It's because you have `from datetime import datetime` and in my example I refer to `timedelta` via a full "module path". Just replace `datetime.timedelta` with `timedelta`. And note that using `eval()` is considered a VERY bad practice in Python, see http://stackoverflow.com/questions/1087255/use-of-eval-in-python for details. – Zaur Nasibov Oct 18 '12 at 08:00

score 1 · Answer 2 · answered Oct 18 '12 at 07:13

1

You are comparing a type and a string. Try this instead:

>>> type(x) == timedelta
True

answered Oct 18 '12 at 07:13

Janne Karila

24,266
6
53
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score 1 · Answer 3 · answered Oct 18 '12 at 07:16

you are comparing string with type , hence it is returning false.

It's best if you use :

 isinstance(x, datetime.timedelta)

See this piece of code, it clearly tells you how to check for type :

    >>> foo = {}

    >>> type(foo)
    <type 'dict'>

    >>> class MyDict(dict):
    ...     pass

    >>> bar = MyDict()

    >>> type(bar)
    <class '__main__.MyDict'>

    >>> type(bar) == dict
    False                     # Unexpected result

    >>> isinstance(bar, dict)
    True                      # Expected result

score 0 · Answer 4 · answered Oct 18 '12 at 07:14

0

the type of 'datetime.timedelta' is str. I think you mean to do if type(x) == timedelta:, which will do the correct typecheck.

answered Oct 18 '12 at 07:14

stein

180
7

score 0 · Answer 5 · answered Oct 18 '12 at 07:41

brief answer(I think this is what you want just for this question):

1.you import the timedelta,and it's name timedelta._name_ is a string 'timedelta'.

2.x = eval(string),here x's type is timedelta which you imported aboved.

so, it's simple,just compare type(x)._name_ with timedelta._name_ if you just want compare them with string :)

Convert string variables into other types

5 Answers5