Why does this code fail? assigning variable to list of itself in for loop
a = 'hello'
b = None
c = None
for x in [a, b, c]:
if isinstance(x, (basestring, dict, int, float)) or x is None:
x = [x]
a
returns 'hello'
, but expected ['hello']
. However, this works:
a = 'hello'
a = [a]
a
returns ['hello']
.
Solution 1:
To achieve that, first you have to understand that you have two diferent refecerences, a and x (for each element), and the reference for the list [a,b,c], used only in the for loop, and never more.
To achieve your goal, you could do this:
a = 'hello'
b = None
c = None
lst = [a, b, c] #here I create a reference for a list containing the three references above
for i,x in enumerate(lst):
if isinstance(x, (str,dict, int, float)) or x is None: #here I used str instead basestring
lst[i] = [x]
print(lst[0]) #here I print the reference inside the list, that's why the value is now showed modified
['hello']
but as I said, if you do print(a) it will show again:
'hello' #here i printed the value of the variable a, wich has no modification
because you never did anything with it.
Take a look at this question to understand a little more about references How do I pass a variable by reference?
Solution 2:
Lets work through this one line at a time;
a = 'hello' # assigns 'hello' to a.
b = None # etc...
c = None
for x in [a, b, c]: # Creates list from the values in 'a' 'b' and 'c', then iterates over them.
# This if statement is always True. x is always either a string, or None.
if isinstance(x, (basestring, dict, int, float)) or x is None:
x = [x] # On the first iteration, sets variable to the expected ['hello']
# After that though - it replaces it with [None]
a # = 'hello' - you have not assigned anything to the variable except on the first line.
The only variable that is ever set to ['hello']
is x
, which is quickly overwritten with None
. If you changed your if check to exclude or x is None
and assigned to a
instead of x
you would get your desired result.
It's also worth noting that the list [a, b, c]
is created when you start the for loop. Changing a
b
or c
during the for loop will have no effect - the list has already been created.