Abstract

Noneshould be a callable object that when called with any arguments has no side effect and returnsNone.

BDFL Pronouncement

This PEP is rejected. It is considered a feature thatNoneraises an error when called. The proposal falls short in tests for obviousness, clarity, explicitness, and necessity. The provided Switch example is nice but easily handled by a simple lambda definition. See Python -dev discussion on 17 June 2005[1].

Motivation

To allow a programming style for selectable actions that is more in accordance with the minimalistic functional programming goals of the Python language.

Rationale

Allow the use ofNonein method tables as a universal no effect rather than either (1) checking a method table entry againstNone before calling, or (2) writing a local no effect method with arguments similar to other functions in the table.

The semantics would be effectively:

classNone:

def__call__(self,*args):
pass

How To Use

Before, checking function table entry againstNone:

classSelect:

defa(self,input):
print'a'

defb(self,input):
print'b'

defc(self,input):
print'c'

def__call__(self,input):
function={1:self.a,
2:self.b,
3:self.c
}.get(input,None)
iffunction:returnfunction(input)

Before, using a local no effect method:

classSelect:

defa(self,input):
print'a'

defb(self,input):
print'b'

defc(self,input):
print'c'

defnop(self,input):
pass

def__call__(self,input):
return{1:self.a,
2:self.b,
3:self.c
}.get(input,self.nop)(input)

After:

classSelect:

defa(self,input):
print'a'

defb(self,input):
print'b'

defc(self,input):
print'c'

def__call__(self,input):
return{1:self.a,
2:self.b,
3:self.c
}.get(input,None)(input)

References

Copyright

This document has been placed in the public domain.