Python accessing parent variable inside nested function

Question

def download(self):
    ftp = self.connect()
    try:
        size = ftp.size(filename=self.filename)
        print "Total size of {filename} is {size}".format(filename=self.filename, size=size)
        written_till_now = 0
        def handle(bytes):
            f.write(bytes)
            print "written bytes: {}".format(bytes)

        with open(self.filename, 'wb') as f:
            ftp.retrbinary('RETR {}'.format(self.filename), handle)
    except (socket.error, error_perm) as e:
        raise DownloaderException("Error in downloading file {filename}: {message}".format(filename=self.filename, message=str(e)))
    except EOFError as e:
        print "file {filename} downloaded successfully".format(filename=self.filename)

I want to keep track of the amount of data i've currently downloaded and later do some additional stuff also for every stream of data i download.

I created a handle function inside the download function.

Python ftblip.retrbinary provides the data to the given callback. In my case handle but somehow it is not executing.

Also I doubt i am not understanding the scope of variables when it comes to nested functions. In my understanding I can use variable defined in parent scope in the child scope as long as i am not modifying them.But in this case f is a object and i am not modifying it rather calling its method write. Please correct me if i am missing something.

I am putting it insie because later i will build a progressbar which will take. so the variable ```written_till_now``` i will access and increment — Raheel, Oct 30 '17 at 12:45
I think its because i am not even getting a single byte. It throws exception o connection refused after a while. — Raheel, Oct 30 '17 at 12:45
I would put it inside the `with` block because you are using `f` variable which is defined there. — Aleksandr Borisov, Oct 30 '17 at 12:48
are you able to get a successful ftp connection ? try to print something before and after `ftp = self.connect()` , if not [this](https://stackoverflow.com/a/2779304/5585424) might help — Ankush Rathi, Oct 30 '17 at 12:52
@AnkushRathi That line is okay because it prints my message `Total size of abc.csv is 88009` — Raheel, Oct 30 '17 at 12:54

score 1 · Accepted Answer · answered Oct 30 '17 at 12:55

1

The handle function doesn't have access to the scoped f variable. Instead you can pass the open file directly to handle.

def handle(fp, bytes):
    fp.write(bytes)
    print "written bytes: {}".format(bytes)

with open(self.filename, 'wb') as f:
    ftp.retrbinary('RETR {}'.format(self.filename), lambda b: handle(f, b))

answered Oct 30 '17 at 12:55

Håken Lid

22,318
9
52
67

```lambda b:``` means that the ftp will use the ```b``` variable ? – Raheel Oct 30 '17 at 12:58
1

It means that the callback function takes a single argument, which we call `b`. – Håken Lid Oct 30 '17 at 13:06

Python accessing parent variable inside nested function

1 Answers1