Python Key Error With _

Question

I'm getting a weird key error with Python dicts. My key is "B19013_001E" and I've named my dict "sf_tracts" with a nested dict "properties". Here is my code:

x = "B19013_001E"
for tract in sf_tracts:
    print tract["properties"][x]

With this, I get a KeyError: "B19013_001E"

However if I change the code to this, the values get printed:

x = "B19013_001E"
for tract in sf_tracts:
    for key in tract["properties"]:
        if key == "B19013_001E":
            print tract["properties"][x]

What's the difference?

-edit- I believe the issue is the underscore as other keys can be printed. How do I access this key?

Thanks

There is nothing special about underscores in strings. What does print list(tract['properties']) produce? Can you share the exact quoted representation of that one key? — Martijn Pieters
– Martijn Pieters, Commented Aug 24, 2016 at 16:41
You have more than one dictionary in sf_tracts. Not all those dictionaries have that key. Your for key in dictionary loop can be replaced with if x in tract["properties"]: print tract["properties"][x], by the way, no need to loop. — Martijn Pieters
– Martijn Pieters, Commented Aug 24, 2016 at 16:42
@MartijnPieters oh, I believe that is the issue, let me check — Jasoniful
– Jasoniful, Commented Aug 24, 2016 at 16:43

Martijn Pieters · Accepted Answer · 2016-08-24 16:45:23Z

3

You are assuming that the key exists in all dictionaries that the tract in sf_tracts loop produces. That assumption is incorrect.

Your second piece of code happens to work because you are essentially testing for the key to exist, albeit expensively. You could instead do this:

for tract in sf_tracts:
    if x in tract["properties"]:
        print tract["properties"][x]

or you could use:

for tract in sf_tracts:
    print tract["properties"].get(x, 'Key is not present')

There is otherwise nothing special about a string key with an underscore in the value. An underscore makes no difference to how such keys are treated.

answered Aug 24, 2016 at 16:45

Martijn Pieters

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Comments

Kevin London · Accepted Answer · 2016-08-24 16:45:31Z

1

Some of your tracts must be missing that particular key. In the first case, you're asking every tract to print the key, while in the second you're limiting the print operation to only those that have the key.

answered Aug 24, 2016 at 16:45

Kevin London

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