4 ms·

Yes and the fact that they aren’t full closures is a travesty - I can’t tell you how many times a nice dependency injection has been ruined by having to add a b

by mlazos 8y ago

Yes and the fact that they aren’t full closures is a travesty - I can’t tell you how many times a nice dependency injection has been ruined by having to add a bunch of def’s. It bloats code and reduces expressiveness.

quietbritishjim 8y ago
Nested functions in Python are "full closures" in every sense I understand it. Is there something in missing? They can read the value of local variables (including arguments) in the parent function, and write to them (using the "nonlocal" statement). If one nested function assigns a new value to a captured variable then, of course, this change is seen by all other nested functions and the enclosing function. Most importantly, if the lifetime of the nested function object is longer than the runtime of the enclosing function (most commonly because you return the nested function object from the enclosing function) then the lifetime of the enclosing function's local variables is extended appropriately. This applies to both nested functions defined using a "def" statement and nested functions defined using a "lambda" expression (except you can't assign to variables in a lambda expression). I say this because you don't say what you mean by "they", and it's a common misconception that only lambda expressions capture enclosing variables. The only disadvantage of this is that, because name lookup is done at execution time rather than parsing time, all the enclosing variables must be captured and their lifetimes extended even if the nested function doesn't use any of them. In this sense, nested functions are too closurey in Python! This is not really so bad because if you don't need variable capture then you should simply not be using a nested function in the first place; this is best anyway because it means you are sharing one function object between different uses.
- mlazos 8y ago
  Sorry if this wasn't clear - I'm talking about lambdas. I'm basically complaining about a piece you pointed out, that you can't assign to variables in a lambda expression, this is very not ergonomic IMO. Having to create a def every time I want a function to assign to a variable is annoying. I pretty much want anonymous nested functions.

by mlazos 8y ago

quietbritishjim 8y ago

Nested functions in Python are "full closures" in every sense I understand it. Is there something in missing? They can read the value of local variables (including arguments) in the parent function, and write to them (using the "nonlocal" statement). If one nested function assigns a new value to a captured variable then, of course, this change is seen by all other nested functions and the enclosing function. Most importantly, if the lifetime of the nested function object is longer than the runtime of the enclosing function (most commonly because you return the nested function object from the enclosing function) then the lifetime of the enclosing function's local variables is extended appropriately. This applies to both nested functions defined using a "def" statement and nested functions defined using a "lambda" expression (except you can't assign to variables in a lambda expression). I say this because you don't say what you mean by "they", and it's a common misconception that only lambda expressions capture enclosing variables. The only disadvantage of this is that, because name lookup is done at execution time rather than parsing time, all the enclosing variables must be captured and their lifetimes extended even if the nested function doesn't use any of them. In this sense, nested functions are too closurey in Python! This is not really so bad because if you don't need variable capture then you should simply not be using a nested function in the first place; this is best anyway because it means you are sharing one function object between different uses.

mlazos 8y ago
Sorry if this wasn't clear - I'm talking about lambdas. I'm basically complaining about a piece you pointed out, that you can't assign to variables in a lambda expression, this is very not ergonomic IMO. Having to create a def every time I want a function to assign to a variable is annoying. I pretty much want anonymous nested functions.