Python if a var doesn't exist then

Question

I'm developing some IMAP checker. Now the inbox count is prints a message in the following layout: ['number'].

Now that that number is split to the number in var num. See the following code:

for num in data[0].split():
    print num

Now the thing is, if there ain't any new emails num doesn't exist so I want an if statement like this:

if <num doesn't exist>: print "No new emails found."

But what should that if statement look like?

Is data the name of your list containing numbers - [number]? And please post the real list what it looks like. — Rohit Jain, Dec 06 '12 at 13:10

score 6 · Answer 1 · answered Dec 06 '12 at 13:10

6

Check this site, they have a useful snippet to check if the variable exists or if it is None:

# Ensure variable is defined
try:
   num
except NameError:
   num = None

# Test whether variable is defined to be None
if num is None:
    some_fallback_operation()
else:
    some_operation(num)

answered Dec 06 '12 at 13:10

alemangui

3,571
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33

why not just - `if num: some_operation(num) else: some_fallback_operation()`? – Rohit Jain Dec 06 '12 at 13:12
I decided to add both in case he wants to make a difference between an undefined variable and a None variable. Your solution also works though. – alemangui Dec 06 '12 at 13:14

bereal · Accepted Answer · 2012-12-06T15:31:37.973

6

The most pythonic way to achieve what you seem to want to, is:

nums = data[0].split()
for num in nums:
   print num
if not nums:
   print "No new emails found"

since the code reflects the intention precisely.

edited Dec 06 '12 at 15:31

answered Dec 06 '12 at 13:20

bereal

32,519
6
58
104

1

Even though the edit was approved, I still think that `if-else` was a more readable option. Purely matter of personal taste, though. – bereal Dec 06 '12 at 14:47

kahowell · Answer 3 · 2012-12-06T13:40:03.047

1

Check the size of the split data and use that as your condition:

for num in data[0].split():
    print num
if len(data[0].split()) == 0:
    print "No new emails found."

More elegant way:

for num in data[0].split():
    print num
if not data[0].split():
    print "No new emails found."

edited Dec 06 '12 at 13:40

answered Dec 06 '12 at 13:14

kahowell

27,286
1
14
9

There are many more ways to do the check. See [this question](http://stackoverflow.com/questions/53513/python-what-is-the-best-way-to-check-if-a-list-is-empty). – kahowell Dec 06 '12 at 13:17
Top example fixed now (had misplaced parenthesis and missing colon), in case someone comes back to this. Bottom example worked for me. – kahowell Dec 06 '12 at 13:43

score 1 · Answer 4 · answered Dec 06 '12 at 13:24

I would do

num = None
for num in data[0].split(): 
    print num
if num is None:
    print "No new emails found."

If None is a valid data portion, use

num = sentinel = object()
for num in data[0].split(): 
    print num
if num is sentinel:
    print "No new emails found."

instead.

Python if a var doesn't exist then

4 Answers4