Basically, sometimes you do want to reuse the mutable between function calls, and in those cases it can save a fair bit of code passing it in repeatedly.
def calculate(a, b, c, memo={}):
try:
value = memo[a, b, c] # return already calculated value
except KeyError:
value = heavy_calculation(a, b, c)
memo[a, b, c] = value # update the memo dictionary
return valueYou can use a mutable default argument as an ersatz static variable, e.g. for memoization.
Maybe I misunderstood you, but they are perfectly mutable:
class A
attr_accessor :a
def initialize
@a = []
end
def b x = a
x << 1
end
end
obj = A.new
# => #<A:0x007fd92b2e9850 @a=[]>
obj.b
# => [1]
obj.b
# => [1, 1]
obj.a
# => [1, 1]
a = 0
# => 0
x = lambda {|y = (a + 1)| y }
# => #<Proc:0x007fd92b1dae50@(irb):33 (lambda)>
x[]
# => 1
a = 5
# => 5
x[]
# => 6 ruby> class A; end
=> nil
ruby> def foo(bar = A.new); return bar; end
=> nil
ruby> foo
=> #<A:0x00000101985060>
ruby> foo
=> #<A:0x0000010197a6b0>
I've even seen a major Python library with this bug (I'm sorry, I don't recall which off-hand). It's really surprising behavior for new Python devs.
>>> def f(x):
... return x + 5
>>> type(f)
<type 'function'>
>>> dis.dis(f.func_code)
2 0 LOAD_FAST 0 (x)
3 LOAD_CONST 1 (5)
6 BINARY_ADD
7 RETURN_VALUE
>>> g = f
>>> g(10)
15
>>> g is f
True
>>> import socket
>>> socket.gethostbyname('www.google.com')
'74.125.71.103'
>>> socket.gethostbyname = lambda i: '10.0.0.1'
>>> socket.gethostbyname('www.google.com')
'10.0.0.1'
Functions just being variables has useful properties when you're doing something like fancy switch/case type things (the readability of this is questionable, but it's cool to look at it, like a Duff's device):
>>> i = 1
>>> { str: func1, unicode: func2, int: func3 }.get(type(i), func1)(i)
in func3
>>> def maker(i):
... def ret(x):
... return x + i
... return ret
>>> f, g = maker(10), maker(100)
>>> f(5), g(10)
(15, 110)
By useful he means mucking up your program in totally unexpected ways.
He's being nice about it being a silly decision to have it behave that way. The entire post reads more like a list of unexpected things that will bite you in the ass.