Unexpected output from list.append()

Question

I am puzzled:

class lin_reg:

     def __init__(self):
    ''' Executes the program '''

    Indep_Array, Dep_Array = self.Prob_Def()
    Total_Array = Indep_Array.append(Dep_Array)
    print Indep_Array, Dep_Array, Total_Array
    NumArray = len(Total_Array)

def Prob_Def(self):

    Analy_Type = raw_input('Type of Regression(linear-n,nonlinear-nl): ')
    Num_IndepVar = eval(raw_input('Number of Independent Variables: '))
    Indep_Array = []
    for IndepVar in range(Num_IndepVar):
        ArrayInput = eval(raw_input('Enter the array: '))
        Indep_Array.append(ArrayInput)
    Dep_Array = eval(raw_input('Enter the dependent array: '))
    return Indep_Array, Dep_Array

When I run this code, I get output as follows:

obs=lin_reg.lin_reg()
Type of Regression(linear-n,nonlinear-nl): nl
Number of Independent Variables: 3
Enter the array: [1,2,3]
Enter the array: [2,3,4]
Enter the array: [3,4,5]
Enter the dependent array: [5,6,7]
[[1, 2, 3], [2, 3, 4], [3, 4, 5], [5, 6, 7]] [5, 6, 7] None

Traceback (most recent call last):
  File "<pyshell#18>", line 1, in <module>
    obs=lin_reg.lin_reg()
  File "C:Python27DataAnalysislin_reg.py", line 13, in __init__
    NumArray=len(Total_Array)
TypeError: object of type 'NoneType' has no len()

How is the dependent array Dep_Array automatically appended to Indep_Array and why is Total_Array returning None?

I was expecting to see output like this for above input:
[[1,2,3],[2,3,4],[3,4,5]] [5,6,7] [[1,2,3],[2,3,4],[3,4,5],[5,6,7]]

Asked By: Jack_of_All_Trades

||

Source

Answer 1

.append modifies the original list and returns None. You want +.

I’ve taken the liberty of rewriting your code to how Python should look.

class lin_reg(object):
     def __init__(self):
         self.indep, self.dep = self.initialise_from_user()
         self.total = self.indep + self.dep
         print self.indep, self.dep, self.total
         self.n = len(self.total)

def initialise_from_user(self):
    analysis_type = raw_input('Type of Regression(linear-n,nonlinear-nl): ')
    n = int(raw_input('Number of Independent Variables: '))

    indep = [np.matrix(raw_input('Enter the array: ')) for _ in range(self.n)]
    dep =  np.matrix(raw_input('Enter the dependent array: '))

    return indep, dep

Answered By: Katriel

Answer 2

It’s because list.append() doesn’t return anything, it changes the list in place. Therefore,

 Total_Array=Indep_Array.append(Dep_Array)

makes Total_Array equal None.

Also, naming conventions are against capitalized variable names.

Answered By: Lev Levitsky

Answer 3

The reason that Dep_Array is automatically appended to Indep_Array is because Indep_Array.append(Dep_Array) modifies Indep_Array.
The function append returns None instead of a list. This was done on purpose to prevent people from chaining functions that modify the arguments, such as a.append(b).remove(c).doSomething(d). By preventing this, the code remains clear and easier to understand for someone unfamiliar with those methods.

Answered By: Simeon Visser

Unexpected output from list.append()

Question:

Answers: