How to access instance variables within string interpolations?

Question

I would like to use string interpolation within instance methods: but am unsure how to access instance variables.

Here is the basic class declaration including the constructor:

class Validation:
    def __init__(self, name, type, count, script):
        self.name = name
        self.type = type
        self.count = count
        self.script = script

And here is where we want to access the list of variables. Note "globals()" is not correct..

    def __repr__(self):
        return "%(self.name)s: %(self.type)s that expects %(self.count)s Rows [%(self.script)s]" %self.__dict__

But that gives exception

(<type 'exceptions.KeyError'>, KeyError('self.name',), None)

So the question is: what should be used in place of %self.dict here?

You can omit the self.: "%(name)s: %(type)s that expects %(count)s Rows [%(script)s]" % self.__dict__. — alecxe
– alecxe, Commented Mar 21, 2014 at 20:06
what @alecxe said. also vars(self) is a little nicer on the eyes than self.__dict__. — roippi
– roippi, Commented Mar 21, 2014 at 20:08
Why not just return "%s: %s that expects %s Rows [%s]" % (self.name, self.type, self.count, self.script)? It's as short as what you've already written and more refactor-friendly. — user2357112
– user2357112, Commented Mar 21, 2014 at 20:08
Avoid using __dict__ directly. If one of those is a property instead of an instance attribute (or if the class uses __slots__), the variable won't be in __dict__. This is also true of using vars, actually. It really is best to just explicitly pass in the variables you want to use. — Joe Kington
– Joe Kington, Commented Mar 21, 2014 at 20:09
@alecxe Pls make that an answer and I will accept it (with roippi's tweak as well) — WestCoastProjects
– WestCoastProjects, Commented Mar 22, 2014 at 0:33

Adam Smith · Accepted Answer · 2014-03-21 20:18:33Z

First of all, string interpolation has been replaced by string formatting, so let's update that:

"{self.name}: {self.type} that expects {self.count} Rows [{self.script}]".format(**self.__dict__)

Second of all, since you're giving it the dict to work with, the keys in the dict are just the variable names, not self.varname.

"{name}: {type} that expects {count} Rows [{script}]".format(**self.__dict__)

Third of all, __repr__ is intended to be valid-ish Python code to reproduce the object. Maybe you meant to put this as __str__? The __repr__ should be

"Validation({name},{type},{count},{script})".format(**self.__dict__)

or honestly, since that's bad practice, just:

"Validation({},{},{},{})".format(self.name,self.type,self.count,self.script)

Let's try it:

#DEMO
class Validation(object):
    def __init__(self, name, type_, count, script):
        self.name = name
        self.type = type_
        self.count = count
        self.script = script
    def __repr__(self):
        return "Validation({},{},{},{})".format(self.name,self.type,self.count,self.script)
    def __str__(self):
        return "{}: {} that expects {} Rows [{}]".format(self.name,self.type,self.count,self.script)

>>> v = Validation('MyName','integer',26,lambda x: x)
>>> print(v) # or str(v)
MyName: integer that expects 26 Rows [<function <lambda> at 0x02D75468>]
>>> repr(v)
'Validation(MyName,integer,26,<function <lambda> at 0x02D75468>)'

alecxe · Accepted Answer · 2014-03-22 01:13:25Z

1

You should omit the self. and use vars() instead of __dict__ (thanks to @roippi):

class Validation:
    def __init__(self, name, type, count, script):
        self.name = name
        self.type = type
        self.count = count
        self.script = script

    def __repr__(self):
        return "%(name)s: %(type)s that expects %(count)s Rows [%(script)s]" % vars(self)


validation = Validation('name', 'type', 'count', 'script')
print validation

prints:

name: type that expects count Rows [script]

answered Mar 22, 2014 at 1:13

alecxe

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