Python, Evaluate a Variable value as a Variable...

Question

Hi!

I'd like to do something like below: particularly the 'f.eval(field)' part, such that it evaluates the value of the variable as the field name. How does one accomplish this in Python?

def punctuated_object_list(objects, field):
    field_list = [f.eval(field) for f in objects]
    if len(field_list) > 0:
        if len(field_list) == 1:
            return field_list[0]
        else:
            return ', '.join(field_list[:-1]) + ' & ' + field_list[-1]
    else:
        return u''

Answer 1

getattr(f, field), if I understand you correctly (that is, if you might have field = "foo", and want f.foo). If not, you might want to clarify. Python has an eval(), and I don't know what other languages' eval() you want the equivalent of.

Answer 2

The python equivalent of eval() is eval()

x = 9
eval("x*2")

will give you 18.

v = "x"
eval(v+"*2")

works too.

Answer 3

To get at a list of all the fields in a Python object you can access its __dict__ property.

class Testing():
    def __init__(self):
        self.name = "Joe"
        self.age = 30

test = Testing()
print test.__dict__

results:

{'age': 30, 'name': 'Joe'}

Answer 4

getattr( object, 'field' ) #note that field is a string

f = 'field_name'
#...
getattr( object, f )


#to get a list of fields in an object, you can use dir()
dir( object )

For more details, see: http://www.diveintopython.org/power_of_introspection/index.html

Don't use eval, even if the strings are safe in this particular case! Just don't get yourself used to it. If you're getting the string from the user it could be malicious code.

Murphy's law: if things can go wrong, they will.