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Using Properties to Validate
Our first coding uses properties to manage three attributes. As usual, we could use simple methods instead of managed attributes, but properties help if we have been using attributes in existing code already. Properties run code automatically on attribute access, but are focused on a specific set of attributes; they cannot be used to intercept all attributes generically.
To understand this code, it’s crucial to notice that the attribute assignments inside the __init__ constructor method trigger property setter methods too. When this method assigns to self.name, for example, it automatically invokes the setName method, which transforms the value and assigns it to an instance attribute called __name so it won’t clash with the property’s name.
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This renaming (sometimes called name mangling) is necessary because properties use common instance state and have none of their own. Data is stored in an attribute called __name, and the attribute called name is always a property, not data.
In the end, this class manages attributes called name, age, and acct; allows the attribute addr to be accessed directly; and provides a read-only attribute called remain that is entirely virtual and computed on demand. For comparison purposes, this property-based coding weighs in at 39 lines of code:
class CardHolder:
acctlen = 8 # Class data
retireage = 59.5
def __init__(self, acct, name, age, addr):
self.acct = acct # Instance data
self.name = name # These trigger prop setters too
self.age = age # __X mangled to have class name
self.addr = addr # addr is not managed
# remain has no data
def getName(self):
return self.__name
def setName(self, value):
value = value.lower().replace(' ', '_')
self.__name = value
name = property(getName, setName)
def getAge(self):
return self.__age
def setAge(self, value):
if value < 0 or value > 150:
raise ValueError('invalid age')
else:
self.__age = value
age = property(getAge, setAge)
def getAcct(self):
return self.__acct[:-3] + '***'
def setAcct(self, value):
value = value.replace('-', '')
if len(value) != self.acctlen:
raise TypeError('invald acct number')
else:
self.__acct = value
acct = property(getAcct, setAcct)
def remainGet(self): # Could be a method, not attr
return self.retireage - self.age # Unless already using as attr
remain = property(remainGet)
Self-test code
The following code tests our class; add this to the bottom of your file, or place the class in a module and import it first. We’ll use this same testing code for all four versions of this example. When it runs, we make two instances of our managed-attribute class and fetch and change its various attributes. Operations expected to fail are wrapped in try statements:
bob = CardHolder('1234-5678', 'Bob Smith', 40, '123 main st')
print(bob.acct, bob.name, bob.age, bob.remain, bob.addr, sep=' / ')
bob.name = 'Bob Q. Smith'
bob.age = 50
bob.acct = '23-45-67-89'
print(bob.acct, bob.name, bob.age, bob.remain, bob.addr, sep=' / ')
sue = CardHolder('5678-12-34', 'Sue Jones', 35, '124 main st')
print(sue.acct, sue.name, sue.age, sue.remain, sue.addr, sep=' / ')
try:
sue.age = 200
except:
print('Bad age for Sue')
try:
sue.remain = 5
except:
print("Can't set sue.remain")
try:
sue.acct = '1234567'
except:
print('Bad acct for Sue')
Here is the output of our self-test code; again, this is the same for all versions of this example. Trace through this code to see how the class’s methods are invoked; accounts are displayed with some digits hidden, names are converted to a standard format, and time remaining until retirement is computed when fetched using a class attribute cutoff:
12345*** / bob_smith / 40 / 19.5 / 123 main st
23456*** / bob_q._smith / 50 / 9.5 / 123 main st
56781*** / sue_jones / 35 / 24.5 / 124 main st
Bad age for Sue
Can't set sue.remain
Bad acct for Sue
