Question

I am a n00b to Python and am attempting to bring the little bit of knowledge I have from shell and PHP scripting to Python. I really trying to grasp the concepts of creating and manipulating the values within (while keeping the code in an understandable form).

I am having trouble utilizing the Python implementations of LISTS and MAPPINGS ( dict() ). I am writing a script that needs to use an associative array (a python mapping) inside of a basic array (a Python list). The list can use the typical INT index.

Thanks!

Here is what I have currently:

'''  Marrying old-school array concepts
[in Python verbiage] a list (arr1) of mappings (arr2)
[per my old-school training] a 2D array with 
        arr1 using an INT index
        arr2 using an associative index
'''
arr1 = []
arr1[0] = dict([    ('ticker'," "),
                        ('t_date'," "),
                        ('t_open'," "),
                        ('t_high'," "),
                        ('t_low'," "),
                        ('t_close'," "),
                        ('t_volume'," ")
                        ] )
arr1[1] = dict([    ('ticker'," "),
                        ('t_date'," "),
                        ('t_open'," "),
                        ('t_high'," "),
                        ('t_low'," "),
                        ('t_close'," "),
                        ('t_volume'," ")
                        ] )

arr1[0]['t_volume'] = 11250000
arr1[1]['t_volume'] = 11260000

print "\nAssociative array inside of an INT indexed array:"
print arr1[0]['t_volume'], arr1[1]['t_volume']

In PHP, I have the following example working:

'''
arr_desired[0] = array( 'ticker'        => 'ibm'
                            't_date'        => '1/1/2008'
                            't_open'        => 123.20
                            't_high'        => 123.20
                            't_low'     => 123.20
                            't_close'   => 123.20
                            't_volume'  => 11250000
                        );
arr_desired[1] = array( 'ticker'        => 'ibm'
                            't_date'        => '1/2/2008'
                            't_open'        => 124.20
                            't_high'        => 124.20
                            't_low'     => 124.20
                            't_close'   => 124.20
                            't_volume'  => 11260000
                        );

print arr_desired[0]['t_volume'],arr_desired[1]['t_volume'] # should print>>> 11250000 11260000
'''

Answer 1

Your list and dict literal definitions can be much simplified:

keys = ['ticker', 't_date', 't_open', 't_high', 't_low', 't_close', 't_volume']
arr1 = [
    dict.fromkeys(keys, ' '),
    dict.fromkeys(keys, ' ')
]

I’m using the dict.fromkeys() method to initialize a dict with a sequence of keys, all with a given default value (a one-space string).

When you define an empty list in Python, you cannot simply address elements that don’t exist. Alternatively, use the .append() method to add new elements to a list:

arr1.append({'key': 'value', 'otherkey': 'othervalue'})

The above example uses the {k: v} dict literal notation.

I suspect you would benefit from reading the (excellent) Python tutorial first.