比较和访问字典列表中的值

Question

I'm IT administrator trying to learn some scripting in Python 3.x. so forgive me my lack of knowledge for now. :-)

I have a list of dictionaries ( start_sequence variable). I want to compare key values between neighbouring dictionaries in list, so I wrote this code:

start_sequence = [{'server': 'server1', 'service': 'service1', 'startup': 'Automatic', 'os': 'Windows', 'order': 1}, 
                  {'server': 'server2', 'service': 'service2', 'startup': 'Automatic', 'os': 'Windows', 'order': 1}, 
                  {'server': 'server3', 'service': 'service2', 'startup': 'Automatic', 'os': 'Windows', 'order': 1}, 
                  {'server': 'server4', 'service': 'service3', 'startup': 'Automatic', 'os': 'Windows', 'order': 2}, 
                  {'server': 'server5', 'service': 'service4', 'startup': 'Automatic', 'os': 'Windows', 'order': 3}]

for i in range(0, len(start_sequence) - 1):
    print('i=', i, ':', start_sequence[i]['server'], start_sequence[i]['service'])
    if start_sequence[i]['order'] != start_sequence[i + 1]['order']:
        print('delay')
print('i=', len(start_sequence) - 1, ':', start_sequence[len(start_sequence) - 1]['server'],start_sequence[len(start_sequence) - 1]['service'])

At this stage this is just to print servers and services from the list and to put a 'delay' basing on order key differences (some services will need to wait until previous are up and running).

And to be honest I don't like this loop too much. It works but I bet it could be written better. For example I don't like this line after the loop just to fix 'list index out of range' issue. Do you have any hints how this could be optimized?

Answer 1

An idea could be to use the zip function in Python. This function returns a list of tuples, where the i-th tuple contains the i-th element from each of the argument sequences or iterables. This could help you iterate the elements:

for thiselem,nextelem in zip(li, li[1 : ] + li[ : 1]):

Note that the only difference is that the last iteration this will give you as thislem the last element in you list and as 'nextelement' the very first.

Here is an example on your data:

for thiselem,nextelem in zip(start_sequence, start_sequence[1 : ] + start_sequence[ : 1]):
  print('THIS: ' + str(thiselem))
  print('NEXT: ' + str(nextelem) + '\n')

Result

THIS: {'server': 'server1', 'order': 1} NEXT: {'server': 'server2', 'order': 1}

THIS: {'server': 'server2', 'order': 1} NEXT: {'server': 'server3', 'order': 1}

THIS: {'server': 'server3', 'order': 1} NEXT: {'server': 'server4', 'order': 2}

THIS: {'server': 'server4', 'order': 2} NEXT: {'server': 'server5', 'order': 3}

THIS: {'server': 'server5', 'order': 3} NEXT: {'server': 'server1', 'order': 1}