working with instance attributes and methods

Question

How do I adapt, the checkLength() method, to accept any instance attributes I pass to it? For example in this context, I can check either first or last, without hardcoding it inside the checkLength() method?

Any help would be most appreciated.

class Name:
  def __init__(self, f, l):
    self.first = f
    self.last = l

  def checkLength(self):
    if type(self.first) == str:
      return len(self.first)
    else:
      return False

myName = Name('Sir', 'Mixalot') 
print(myName.checkLength())

I have tried the following combinations:

  def checkLength(self, last):
    if type(self.last) == str:
      return len(self.last)
    else:
      return False

print(myName.checkLength(last))

  def checkLength(self.last):
    if type(self.last) == str:
      return len(self.last)
    else:
      return False

print(myBody.checkLength(self.last))

Update - trying to use getattr

  def __init__(self, f, l):
    self.first = f
    self.last = l

  def checkLength(self, attr):
    if type(getattr(self, attr)) == str:
      return len(getattr(self, attr))
    else:
      return False

myName = Name('Sir', 'Mixalot') 
print(myName.checkLength(self, "last"))

error =

NameError: name 'self' is not defined````

Could you elaborate on exactly what is that you want to achieve with the checkLength function ? As in what is the desired use of this function ? — Tanishq Vyas
– Tanishq Vyas, Commented Dec 14, 2020 at 13:27
Why don't you simply use the len function directly? For example: print(len(myName.last)) Or, if you need a version of len which checks the type first, you can write this function (typeSafeLen) independently of your class. — kol
– kol, Commented Dec 14, 2020 at 13:30
Your question is not clear. According to what are you going to choose between checking the len of f or l? — Tomerikoo
– Tomerikoo, Commented Dec 14, 2020 at 13:38
Side-note: For type-checking, you want either if isinstance(self.first, str): (allows subclasses) or if type(self.first) is str: (strict test for that exact class). Don't use == str; the str class is a singleton, and if you want strict checking, use is for identity testing, not == (for loose value equality testing). — ShadowRanger
– ShadowRanger, Commented Dec 14, 2020 at 13:38

Roman Ferenets · Accepted Answer · 2020-12-14 13:48:21Z

1

Consider using getattr(object, name[, default]). For example:

class Name:
  def __init__(self, f, l):
    self.first = f
    self.last = l

  def checkLength(self, attr):
    if type(getattr(self, attr)) == str:
      return len(getattr(self, attr))
    else:
      return False

myName = Name('Sir', 'Mixalot') 
print(myName.checkLength("first"))

3

But you should also add some error handling when that attribute is missing.

edited Dec 14, 2020 at 13:48

answered Dec 14, 2020 at 13:28

Roman Ferenets

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2 Comments

dimButTries Over a year ago

thank you for your kind response, I am not familiar with getattr, your suggestion is most helpful. I updated my code in the original post. I am still struggling with this solution as I get a error message print(myBody.checkLength(self, last)). NameError: name 'self' is not defined

Roman Ferenets Over a year ago

@dimButTries I thinks you forgot to put all that in the class.

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