If you run this code with Python 2.7,
i = 5
x = [1, 2, 3, 4]
y = [i*i for i in x if i % 2 == 0]
The value of i is changed to 4. But how?
Asked
Active
Viewed 157 times
-3
MetallicPriest
- 29,191
- 52
- 200
- 356
-
python 3 or python 2? – hiro protagonist Aug 20 '19 at 11:55
-
The value is equal to 5 with python 3. – Bando Aug 20 '19 at 11:56
-
Use another variable name in your loop if you use python 2. – Kostas Charitidis Aug 20 '19 at 11:57
-
I can use another variable of course, but I was just asking the reason why it can cause this. – MetallicPriest Aug 20 '19 at 11:57
2 Answers
3
I think you're using Python 2.7, which leaks the variables defined in the comprehension. On the last iteration the i in the loop is equal to 4, and the name i from the comprehensionthen shadows the global name once control leaves the comprehension.
Python 3 doesn't do that anymore, though, so you should update.
ForceBru
- 43,482
- 10
- 63
- 98
2
That's a Python 2 specific limitation. Variables inside list comprehension have their own scope in Python 3:
Python 3.7.2 (default, Mar 11 2019, 11:54:40)
[GCC 7.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> i = 5
>>> x = [1, 2, 3, 4]
>>> y = [i*i for i in x if i == 2]
>>> i
5
jfaccioni
- 7,099
- 1
- 9
- 25