Building a HashMap using Python in a loop. Clarification needed

Question

I am running in a loop and and parsing a string. That string ultimately contains 2 items a host and application running on a host. As expected a host runs multiple applications. I'd like to store it all in one data structure where host is used as a key.

Below is my failed attempt. Please help me understand why only the last element is being saved in the host = app format.

What i expect to see
host = app1, app2 etc
What i see
Host = app2 (always last)


data = dict()

def add(line):
    l = line.split("/")
    host = l[0].strip()
    app = l[-1].strip()

    data[host].append(app)

for entry in env:
    if "/" not in entry: continue
    add(entry)

print data

Possible duplicate of [How does collections.defaultdict work?](http://stackoverflow.com/questions/5900578/how-does-collections-defaultdict-work) — Wayne Werner, Aug 01 '16 at 20:17
This would fail with a `KeyError`, not only return the last item. This REALLY needs a [mcve]. — Morgan Thrapp, Aug 01 '16 at 20:21

James K · Accepted Answer · 2016-08-01T20:08:45.890

1

The problem is with data[host].append(app)

Would append the value of app to the list (or similar) stored as data[host]

When you run this, the value of data[host] is not set, and so you can't append to it. You will get a KeyError. Perhaps you meant data[host] = app? or...

try:
    data[host].append(app)
except KeyError:
    data[host] = [app]

edited Aug 01 '16 at 20:08

answered Aug 01 '16 at 19:01

James K

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Right. Tried that. This sets the last value in the list. If I iterate over 5 elements, only 5th one gets added – James Raitsev Aug 01 '16 at 19:21
You can `try` to append to the list, and if that doesn't work start a new list. – James K Aug 01 '16 at 20:09

score 0 · Answer 2 · answered Aug 01 '16 at 18:57

0

data[host].append(app)

You assume host is in the dict, which is not true for the first time you append to it.

answered Aug 01 '16 at 18:57

Israel Unterman

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This check causes an error `if host in data: data[host].append(app) else: data[host]=app` – James Raitsev Aug 01 '16 at 19:18

score 0 · Answer 3 · answered Aug 01 '16 at 20:46

0

This is what I was looking for. Thank you to all those trying to help

if host in data:
    data[host].append(app)
else:
    data[host] = [app]

answered Aug 01 '16 at 20:46

James Raitsev

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Building a HashMap using Python in a loop. Clarification needed

3 Answers3