setting column names in pandas which has no name before code example

Example: pandas dataframe without column names

Nope, you're not required to assign column names, nor do you need them to access any element.

In [12]: df = pd.DataFrame([0])

In [13]: df.ix[0,0]
Out[13]: 0

In [14]: df[0][0]
Out[14]: 0
In fact, you can think of the column already having a name -- it is the integer 0. Look at what happens when you provide a name

In [15]: df    #Before naming the column
Out[15]:
   0
0  0

In [16]: df.columns = ['ColA']
In [17]: df    #Renamed
Out[17]:
   ColA
0     0

In [18]: df['ColA'][0]    #Now you can access the column using the new name
Out[18]: 0

In [19]: df[0][0]         #... but trying the old name will not work
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)

KeyError: 'no item named 0'
You can still use DataFrame.ix just as before, though:

In [20]: df.ix[0,0]
Out[20]: 0