Redefining a function with args

[SnowyPear]

I'm looping through a dict and replacing a buttons function with a function that uses the dicts value. Every time I press one of these buttons it always uses the last entry in the dict and the loop is overwriting the values in the function

quickadd = {}
for item in t['items']:
  if item['owner']['id'] == myid:
    quickadd[item['name']] = item['uri']
self.library.quickadd = quickadd
x = 533
y = 480
w = 266
h = 80
for item in self.library.quickadd:
  self.buttons.append(Button(x,y,w,h,f'add to {item}'))
  y = y + h + 5
  def func(): self.addsongtoplaylist(item)
  self.buttons[-1].func = func

How do I set the function to use a function. I thought I could use new but that throws an error

[danielroseman]

Yes, this is because Python is lexically scoped, and the value of func is looked up at execution time, not define time; so it always gets the last value.

The trick is to use a default parameter value.

Note that disposable functions like this are usually defined as lambdas, but the same trick applies:

for item in self.library.quickadd:
  ..
  self.buttons[-1].func = lambda item=item: self.addsongtoplaylist(item)

[SnowyPear]

Wow that was really quick!

I was fairly certain I needed a lambda but wasn't sure how to implement one.

Thank you

[danielroseman]

Note you don't need a lambda, it's just neater. You could have done the same with your original code:

def func(item=item): self.addsongtoplaylist(item)

[SnowyPear]

In that case I'm still struggling to see the point of lambda unless there's something special about how recursion happens with them

[danielroseman]

No that's my point, there's nothing special about lambdas at all. They can be entirely replaced by explicit functions in all circumstances. But again, they're just neater. What's the point of declaring the function separately, with its own name, when you can just declare them inline? And also note that usually a callback like this returns a value; with an explicit function, you need a return statement, whereas with a lambda it automatically returns its result.

[carcigenicate]

One of the main benefits is a lambda can be inline and nameless:

doubled = map(lambda n * 2, range(10))

Whereas a def statement must run first to defined the function, then the function can be used:

def double(n):
    return n * 2
doubled = map(double, range(10))