Python list append

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.

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.

The object you are appending (fi) is mutable (see Python documentation), 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.

Another related question is "How do I pass a variable by reference?".

Daren Thomas used assignment to explain how variable passing works in Python. For the append method, we could think in a similar 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 append: ", list_of_variables list_of_values.append(10) list_of_variables.append(list_of_values) print "List of variables after 2nd append: ", 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 the first and second item in "list_of_variables" are pointing to the same object in memory, the output from above is:

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

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