I want to write a __str__ for my class. However, the class has many members and not all may always be relevant. I want the __str__ to include only the key members by default, but include more if the user asks for more.
How can I do this? Can __str__ take parameters (I am okay with giving them default values)? How will these parameters be passed when the user does print(my_class_instance)?
The most common way of doing this in python is to define a __format__ method so that you can use format specifiers much like "{:05.3f}" can format floating point numbers.
If you only need 2 representations, one to be more user friendly and one to be more descriptive you may be able to get away with just defining __str__ and __repr__, for context __repr__ is used when displaying values in an interactive terminal or when calling repr or using "{0!r}".format.
Here is a sample code to get you started:
class Demo:
def __init__(self, a,b):
self.a = a
self.b = b
def __repr__(self):
# should usually have a repr method to show code representation
return "Demo({0.a!r}, {0.b!r})".format(self)
def __str__(self):
# str should give user friendly representation, default format
return "Demo()"
def __format__(self, spec):
# format for more specific details
if spec == "": # usually empty spec should be equivalent to str.
return str(self)
elif spec == "d":
return "(a:{0.a}, b:{0.b})".format(self)
else:
raise NotImplementedError("unsupported spec: {!r}".format(spec))
x = Demo("A", "B")
print("str: {0}, repr: {0!r}, spec='d': {0:d}".format(x))
print("WILL THROW ERROR: {0:OTHER_SPEC}".format(x))
You can make any format spec you wish, what ever is passed into the specifier will be passed to the __format__ method as a string.