Unbound Local Error when Assigning to Function Arg

Question

def make_accumulator(init):
    def accumulate(part): 
        init = init + part 
        return init
    return accumulate

A = make_accumulator(1)
print A(2)

gives me:-

Traceback (most recent call last):
  File "make-accumulator.py", line 8, in <module>
    print A(2)
  File "make-accumulator.py", line 3, in accumulate
    init = init + part 
UnboundLocalError: local variable 'init' referenced before assignment

Why is init not visible inside accumulate?

where did you initialised init... when interpreter wants to do init=init+part, it should know init initialised before.. — Lokesh Sanapalli, Sep 14 '16 at 08:42
use a constructor to set the init value... ` def make_accumulator(self,init): def __init__(self,init): self.init=init` — Lokesh Sanapalli, Sep 14 '16 at 08:45
@lokesh1729 They are passing its value to `make_accumulator`, it's a closure variable. If they only do `return init + part` it will work fine. — Ashwini Chaudhary, Sep 14 '16 at 08:49

score 1 · Accepted Answer · answered Sep 14 '16 at 08:43

That's because during parsing the inner function when Python sees the assignment init = init + part it thinks init is a local variable and it will only look for it in local scope when the function is actually invoked.

To fix it add init as an argument to accumulate with default value of init:

def make_accumulator(init):
    def accumulate(part, init=init): 
        init = init + part 
        return init
    return accumulate

Read: Why am I getting an UnboundLocalError when the variable has a value?

score 1 · Answer 2 · edited May 23 '17 at 12:01

>>> def make_accumulator(init):
...     def accumulate(part): 
...         return init + part
...     return accumulate
... 
>>> make_accumulator(1)
<function accumulate at 0x7fe3ec398938>
>>> A(2)
3

Since you declare init inside accumulate, Python interprets it as local and therefore referenced before assignment. (Note that I removed the init = part).

I am definitely not an expert about this, but got the hint from these posts : Here and Here.

I guess someone could explain it better...

Unbound Local Error when Assigning to Function Arg

2 Answers2