Python list append

Question

I want to store the intermediate values of a variable in Python. This variable is updated in a loop. When I try to do this with a list.append command, it updates every value in the list with the new value of the variable. How should I do it ?

while (step < maxstep):
    for i in range(100):    
        x = a*b*c
        f1 += x
    f2.append(f1)
    print f2
    raw_input('<<')
    step += 1

Expected output

[array([-2.03,-4.13])]
<<
[array([-2.03,-4.13]),array([-3.14,-5.34])]

Printed output

[array([-2.03,-4.13])]
<<
[array([-3.14,-5.34]),array([-3.14,-5.34])]

Is there a different way of getting what I want in python?

What is the meaning of `condition` and `m`? What is `fi`? What is `f()`? — , Sep 05 '12 at 10:42
X is updated based on a function , m is the range of the for loop, the while loop runs for an arbitrary number of steps — abcd, Sep 05 '12 at 10:48
But now I'm confused, I see that you have `f1` and `fi` is that a typo in your code? — zenpoy, Sep 05 '12 at 10:49

zenpoy · Accepted Answer · 2012-09-05T11:00:17.877

9

Assuming the original had a typo and f1 is actualy fi (or vice verca):

fi is a pointer to an object, so you keep appending the same pointer, when you fi += x you actually changing the value of the object to which fi points. Hope this is clear.

To solve the issue you can fi = fi + x instead.

edited Sep 05 '12 at 11:00

answered Sep 05 '12 at 10:48

zenpoy

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Ah!, I did not realize that there was such a difference between f1 += x and f1=f1+x It solved the issue :) – abcd Sep 05 '12 at 10:57
Added your solution for the next generations – zenpoy Sep 05 '12 at 11:00

score 4 · Answer 2 · answered Sep 05 '12 at 10:50

I suppose you meant something like this:

   f2 = []
   f1 = 0
   for i in range(100):    
       x = f()
       f1 += x
   f2.append(f1)
   print f2

Note that if f1 is a mutable object, the line f1 += x doesn't create new object, but only changes value of f1, so all its occurrences in f2 array are updated.

score 2 · Answer 3 · answered Sep 05 '12 at 10:47

It appears you are appending the same array to the list, and then changing the content of the array

You need to create a new array object each time you append it to f2

You'll need to add more info to the code in your question if you need more help. It doesn't make much sense at the moment (where is the value of fi changed?)

score 1 · Answer 4 · answered Sep 05 '12 at 10:54

You should really paste a working example.

The object you are appending (fi) is mutable (see Python docs), meaning, in essence, that you are appending a reference to the object, not the object value. Therefore, both list index 0 and 1 are actually the same object.

You need to either create a new object (fi = array()) on every loop iteration or use the copy module.

score 1 · Answer 5 · edited May 23 '17 at 11:48

Another related question is "How do I pass a variable by reference?". Daren Thomas used assignment to explain how variable pass works in python. For the append method, we could think in a similarly way. Say you're appending a list "list_of_values" to a list "list_of_variables",

list_of_variables = []
list_of_values = [1, 2, 3]
list_of_variables.append(list_of_values)
print "List of variables after 1st appending: ", list_of_variables
list_of_values.append(10)
list_of_variables.append(list_of_values)
print "List of variables after 2nd appending: ", list_of_variables

The appending operation can be thought as:

list_of_variables[0] = list_of_values --> [1, 2, 3]
list_of_values --> [1, 2, 3, 10]
list_of_variables[1] = list_of_values --> [1, 2, 3, 10]

Because 1st and 2nd item in "list_of_variables" are pointing to the same object in memory, the output from above is:

List of variabiles after 1st appending:  [[1, 2, 3]]
List of variables after 2nd appending:  [[1, 2, 3, 10], [1, 2, 3, 10]]

On the other hand, if "list_of_values" is a variable, the behavior will be different.

list_of_variables = []
variable = 3
list_of_variables.append(variable)
print "List of variabiles after 1st appending: ", list_of_variables
variable = 10
list_of_variables.append(variable)
print "List of variables after 2nd appending: ", list_of_variables

The appending operation now is equivalent to:

list_of_variables[0] = variable --> 3
variable --> 4
list_of_variables[1] = variable --> 4

And the output is:

List of variabiles after 1st appending:  [3]
List of variables after 2nd appending:  [3, 10]

The difference between variable and list_of_values is the later one changes in-place.

Python list append

5 Answers5