"UnboundLocalError: local variable referenced before assignment" after an if statement

Question

When I try this code:

tfile = open("/home/path/to/file",'r') 

def temp_sky(lreq, breq):
    for line in tfile:
        data = line.split()
        if (    abs(float(data[0]) - lreq) <= 0.1 
            and abs(float(data[1]) - breq) <= 0.1):            
            T = data[2]
    return T
print(temp_sky(60, 60))
print(temp_sky(10, -10))

I get an error that says

Traceback (most recent call last):
  File "tsky.py", line 25, in <module>
    print(temp_sky(10, -10))
  File "tsky.py", line 22, in temp_sky
    return T
UnboundLocalError: local variable 'T' referenced before assignment

The first print works correctly but the second causes an exception. I tried making T a global variable but then both answers are the same.

What is going wrong, and how can I fix it?

to get rid of UnboundLocalError, the if statement has to run, so try give T a default value so that T is defined, refer to @shx2 answer — PurityLake, Mar 12 '13 at 17:20
Also, you are running an entire loop to get a single value. This way your loop will always return the last matching instance of data. You can make your code more efficient by reading it in reverse order and instead of assigning, `return T` — ferrix, Mar 12 '13 at 17:28

score 46 · Accepted Answer · edited Nov 06 '21 at 18:33

Your if statement is always false and T gets initialized only if a condition is met, so the code doesn't reach the point where T gets a value (and by that, gets defined/bound). You should introduce the variable in a place that always gets executed.

Try:

def temp_sky(lreq, breq):
    T = <some_default_value> # None is often a good pick
    for line in tfile:
        data = line.split()
        if abs(float(data[0])-lreq) <= 0.1 and abs(float(data[1])-breq) <= 0.1:
            T = data[2]
    return T

score 9 · Answer 2 · answered Nov 24 '14 at 19:33

FWIW: I got the same error for a different reason. I post the answer here not for the benefit of the OP, but for the benefit of those who may end up on this page due to its title... who might have made the same mistake I did.

I was confused why I was getting "local variable referenced before assignment" because I was calling a FUNCTION that I knew was already defined:

def job_fn(job):
  return job + ".job"

def do_something():
  a = 1
  b = 2
  job_fn = job_fn("foo")

do_something()

This was giving:

UnboundLocalError: local variable 'job_fn' referenced before assignment

Took me a while to see my obvious problem: I used a local variable named job_fn which masked the ability to see the prior function definition for job_fn.

Another possible cause for the error. http://stackoverflow.com/questions/10506973/can-not-increment-global-variable-from-function-in-python — digao_mb, May 10 '16 at 11:23
There's actually [an entry in the Python FAQ](https://docs.python.org/3/faq/programming.html#why-am-i-getting-an-unboundlocalerror-when-the-variable-has-a-value) about this, and some other SO questions like [this one](https://stackoverflow.com/q/370357/4518341) — wjandrea, Jun 13 '20 at 03:15

score 5 · Answer 3 · edited Jan 22 '22 at 22:31

The other answers are correct: You don't have a default value. However, you have another problem in your logic:

You read the same file twice. After reading it once, the cursor is at the end of the file, so trying to read it again returns nothing and the loop is never entered. To solve this, you can do two things: Either open/close the file upon each function call:

def temp_sky(lreq, breq):
    with open("/home/path/to/file",'r') as tfile:
        # do your stuff

This has the disadvantage of having to open the file each time. The better way would be:

tfile.seek(0)

You do this after your for line in tfile: loop. It resets the cursor to the beginning so the next call will start from there again.

Related questions:

After my error was resolved I was still not getting the right answer and this was due to the file cursor not being in the right place. I have amended this as you suggested and is now working. Thanks — user1958508, Mar 12 '13 at 18:32

score 1 · Answer 4 · answered Mar 12 '13 at 17:28

Before I start, I'd like to note that I can't actually test this since your script reads data from a file that I don't have.

'T' is defined in a local scope for the declared function. In the first instance 'T' is assigned the value of 'data[2]' because the conditional statement above apparently evaluates to True. Since the second call to the function causes the 'UnboundLocalError' exception to occur, the local variable 'T' is getting set and the conditional assignment is never getting triggered.

Since you appear to want to return the first bit of data in the file that matches your conditonal statement, you might want to modify you function to look like this:

def temp_sky(lreq, breq):
    for line in tfile:
        data = line.split()
        if ( abs(float(data[0]) - lreq) <= 0.1 and abs(float(data[1]) - breq) <= 0.1):            
            return data[2]
    return None

That way the desired value gets returned when it is found, and 'None' is returned when no matching data is found.

That is actually not identical behavior. The example given returns the last matching instance and you are returning the first instance. Reading the file from the end would work but a simple `reversed()` will not do if we don't want the entire file in memory. — ferrix, Mar 12 '13 at 17:31

score 0 · Answer 5 · answered May 11 '15 at 04:26

I was facing same issue in my exercise. Although not related, yet might give some reference. I didn't get any error once I placed addition_result = 0 inside function. Hope it helps! Apologize if this answer is not in context.

user_input = input("Enter multiple values separated by comma > ")

def add_numbers(num_list):
    addition_result = 0
    for i in num_list:
        addition_result = addition_result + i
    print(addition_result)

add_numbers(user_input)

score 0 · Answer 6 · edited Sep 14 '18 at 10:44

Contributing to ferrix example,

class Battery():

    def __init__(self, battery_size = 60):
        self.battery_size = battery_size
    def get_range(self):
        if self.battery_size == 70:
            range = 240
        elif self.battery_size == 85:
        range = 270

        message = "This car can go approx " + str(range)
        message += "Fully charge"
        print(message)

My message will not execute, because none of my conditions are fulfill therefore receiving " UnboundLocalError: local variable 'range' referenced before assignment"

def get_range(self):
    if self.battery_size <= 70:
        range = 240
    elif self.battery_size >= 85:
        range = 270

score -1 · Answer 7 · answered Mar 03 '17 at 04:40

-1

To Solve this Error just initialize that variable above that loop or statement. For Example var a =""

answered Mar 03 '17 at 04:40

Mihit Gandhi

135
10

This is Python, not JS – wjandrea Jun 09 '20 at 20:30

"UnboundLocalError: local variable referenced before assignment" after an if statement

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