language design - "Least Astonishment" in Python: The Mutable Default Argument -

language design - "Least Astonishment" in Python: The Mutable Default Argument -

anyone tinkering python long plenty has been bitten (or torn pieces) next issue:

def foo(a=[]): a.append(5) homecoming

python novices expect function homecoming list 1 element: [5]. result instead different, , astonishing (for novice):

>>> foo() [5] >>> foo() [5, 5] >>> foo() [5, 5, 5] >>> foo() [5, 5, 5, 5] >>> foo()

a manager of mine 1 time had first encounter feature, , called "a dramatic design flaw" of language. replied behavior had underlying explanation, , indeed puzzling , unexpected if don't understand internals. however, not able reply (to myself) next question: reason binding default argument @ function definition, , not @ function execution? uncertainty experienced behavior has practical utilize (who used static variables in c, without breeding bugs?)

edit:

baczek made interesting example. of comments , utaal's in particular, elaborated further:

>>> def a(): ... print "a executed" ... homecoming [] ... >>> >>> def b(x=a()): ... x.append(5) ... print x ... executed >>> b() [5] >>> b() [5, 5]

to me, seems design decision relative set scope of parameters: within function or "together" it?

doing binding within function mean x bound specified default when function called, not defined, nowadays deep flaw: def line "hybrid" in sense part of binding (of function object) happen @ definition, , part (assignment of default parameters) @ function invocation time.

the actual behavior more consistent: of line gets evaluated when line executed, meaning @ function definition.

actually, not design flaw, , not because of internals, or performance. comes fact functions in python first-class objects, , not piece of code.

as think way, makes sense: function object beingness evaluated on definition; default parameters kind of "member data" , hence state may alter 1 phone call other - in other object.

in case, effbot has nice explanation of reasons behavior in default parameter values in python. found clear, , suggest reading improve knowledge of how function objects work.

python language-design least-astonishment