# Example class to represent temperatures in various units. Showcases

# the use of properties to define managed attributes in classes that

# look like normal attributes from the outside, but whose setters and

# getters can silently perform arbitrary computations.

class Temperature:

def __init__(self, t, unit = 'K'):

if unit == 'K':

self.K = t

elif unit == 'C':

self.C = t

elif unit == 'F':

self.F = t

else:

raise ValueError(f"Unknown unit system {unit}!")

# The temperature is actually stored as kelvins inside the object.

# However, the properties K, C and F allow the same temperature

# to be accessed from the outside in different units as desired.

@property

def K(self):

return self.__k

@K.setter

def K(self, k):

# Prevent the creation of impossible temperatures.

if k < 0:

raise ValueError(f"Absolute temperature {k} may not be negative.")

else:

self.__k = k

# The properties C and F are managed, that is, their values are

# not actually stored inside the object, but they are computed

# on the spot every time they are requested. When some class is

# properly designed, it should be impossible for outside users

# to be able to tell which properties are managed.

@property

def C(self):

return self.K - 273

@C.setter

def C(self, c):

self.K = c + 273

@property

def F(self):

return 1.8 * self.C + 32

@F.setter

def F(self, f):

self.C = (f - 32) / 1.8

# The dunder methods for the string representation of object.

# String representation meant for human readers.

def __str__(self):

return f"Temperature of {self.K} kelvins."

# Representation meant for computer and eval function.

def __repr__(self):

return f"Temperature({self.K}, 'K')"

# For temperatures, addition is meaningless, but temperatures

# can be meaningfully subtracted from each other. Weird.

def __sub__(self, other):

return Temperature(abs(self.K - other.K))

# Technically, temperature and temperature difference are two

# separate concepts of the problem domain in physics, and thus

# should be represented by two different data types, analogous

# how datetime and timedelta are different data types in the

# datetime standard library module.

if __name__ == "__main__":

t1 = Temperature(30, 'C')

print(f"Temperature is {t1.C:.1f} C, {t1.F:.1f} F, {t1.K:.1f} K.")

t2 = Temperature(100, 'F')

print(f"Temperature is {t2.C:.1f} C, {t2.F:.1f} F, {t2.K:.1f} K.")

t3 = t1 - t2

print(f"Their difference is {t3.K:.1f} K.")

# And the crash.

try:

t4 = Temperature(-400, 'C')

except ValueError as e:

print(f"Caught error: {e}")