I have a class whose instances need to format output as instructed by the user. There's a default format, which can be overridden. I implemented it like this:
class A:
def __init__(self, params):
# ...
# by default printing all float values as percentages with 2 decimals
self.format_functions = {float: lambda x : '{:.2%}'.format(x)}
def __str__(self):
# uses self.format_functions to format output
# ...
a = A(params)
print(a) # uses default output formatting
# overriding default output formatting
# float printed as percentages 3 decimal digits; bool printed as Y / N
a.format_functions = {float : lambda x: '{:.3%}'.format(x),
bool : lambda x: 'Y' if x else 'N'}
print(a)
Is it ok? Let me know if there is a better way to design this.
Unfortunately, I need to pickle instances of this class. But only functions defined at the top level of the module can be pickled; lambda
functions are unpicklable, so my format_functions
instance attribute breaks the pickling.
I tried rewriting this to use a class method instead of lambda functions, but still no luck for the same reason:
class A:
@classmethod
def default_float_format(cls, x):
return '{:.2%}'.format(x)
def __init__(self, params):
# ...
# by default printing all float values as percentages with 2 decimals
self.format_functions = {float: self.default_float_format}
def __str__(self):
# uses self.format_functions to format output
# ...
a = A(params)
pickle.dump(a) # Can't pickle <class 'method'>: attribute lookup builtins.method failed
Note that pickling here doesn't work even if I don't override the defaults; just the fact that I assigned self.format_functions = {float : self.default_float_format}
breaks it.
What to do? I'd rather not pollute the namespace and break encapsulation by defining default_float_format
at the module level.
Incidentally, why in the world does pickle
create this restriction? It certainly feels like a gratuitous and substantial pain to the end user.
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