How to use for loop inside dictionary

Question

Can anyone please explain the below implementation:

item = dict((i.tag, (type(i.text) == str and i.text.strip() or i.text)) for i in r if i.tag != "tid_item")

The values I am getting in various variables:

r is something like : <Element 'Rows' at 0x0000000003DA4540>
i.tag : resultNum
i : <Element 'resultNum' at 0x0000000003EA45A0>
i.text : 1

I am python novice and I am unable to understand how forloop is used within dictionary as values are also absurd.

Thanks for help!

Answer 1

let's make first the code clearer:

dict(               #  constructor
    (i.tag,         #  key
         (type(i.text) == str and i.text.strip() or i.text)     # value
    ) 
    for i in r                  # driver of the generator
    if i.tag != "tid_item"      # conditional selector
)

What you have here, is not yet a dictionary, but a constructor of a dictionary, using a generator. After this is run, the variable item it is assigned to will contain a dictionary

the for loop inside this constructor is the generator to create all the elements: it loops over all elements in r, if it satisfies the condition, then it will create a tuple ( key, value) -> creating a 'on-the-fly' list of elements.

The boolean selector for the 'value' is also simple if we write it differently:

value = i.text.strip() if (type(i.text) == str) else i.text

Answer 2

Firstly, lets decompose your example:

item_init = ((i.tag, (type(i.text) == str and i.text.strip() or i.text)) for i in r if i.tag != "tid_item")
item = dict(item_init)

Now, if you look at the definition of the type dict in python (help(dict)), you will see that a dict object can be initialized with an iterable of (key, value) pairs. The item_init variable contains a generator and yield an iterable of tuples.

Next, look at the expression (i.tag, (type(i.text) == str and i.text.strip() or i.text)). You may not understand the second part of the expression because it looks like a boolean operation but is actually a conditionnal assignment operation which means:

if type(i.text) is str then assign i.text.strip() else, assign i.text

Finally, the item_init object is a generator of 2-uples where, for each element of r, the first part is the tag and the second is the text (stripped, if necessary). The tag will be used as keys and the text as values in the final dict object.