Recently I started playing around with Python and I came around something peculiar in the way closures work. Consider the following code:
adders= [0,1,2,3] for i in [0,1,2,3]: adders[i]=lambda a: i+a print adders(3)
It builds a simple array of functions that take a single input and return that input added by a number. The functions are constructed in for loop where the iterator i runs from 0 to 3. For each of these number a lambda funciton is created which captures i and adds it to the function's input. The last line calls the second lambda function with 3 as a parameter. To my surprise the output was:
I expected a 4. My reasoning was: in Python everything is an object and thus every variable is essential a pointer to it. When creating the lambda closures for i, I expected it to store a pointer to the integer object currently pointed to by i. That means that when i assigned a new integer object it shouldn't effect the previously created closures. Sadly, inspecting the adders array within a debugger shows that it does. All lambda functions refer to the last value of i, 3, which results in adders1 returning 6.
Which me the following:
- what does the closures capture exactly?
- What is the most elegant way to convince that lambda functions to capture the current value of i and in a way that will not be affected when i changes it's value.