Python: try except KeyError vs if has_key()

Question

In the following code, what's more efficient / more 'Pythonic'? using a try-catch clause or a if-else clause?

fname = 'AdobeARM.log'

letters = {}

with open(fname,'r') as f:
    for line in f:
        for c in line:
            try:
                letters[c] += 1
            except KeyError:
                letters[c] = 1

print letters

VS.

fname = 'AdobeARM.log'

letters = {}

with open(fname,'r') as f:
    for line in f:
        for c in line:
            if letters.has_key(c):
                letters[c] += 1
            else:
                letters[c] = 1

print letters

I tend to go with the try catch option, but I'm not sure why.

IMO, second way is more self-explaining. And try-catch must have an overhead. — Seçkin Savaşçı, Sep 09 '12 at 20:50
see http://stackoverflow.com/questions/12342207/count-frequency-of-letters-in-a-text-file. — Ashwini Chaudhary, Sep 09 '12 at 21:03
Noone mentions membeship testing via "in". "if c in letters:" reads much more cleanly to me IMHO. — meawoppl, Feb 20 '14 at 04:55

Ashwini Chaudhary · Answer 1 · 2016-08-24T05:21:28.617

23

Use dict.get():

get(key[, default])

Return the value for key if key is in the dictionary, else default. If default is not given, it defaults to None, so that this method never raises a KeyError.

In other words d.get('x', c) is equivalent to d['x'] if 'x' in d else c.

Example:

In [24]: d = {'a':1, 'b':2}

In [27]: d['d'] = d.get('d', 0) + 1  # 0 is the default value

In [28]: d
Out[28]: {'a': 1, 'b': 2, 'd': 1}

In [29]: d['d'] = d.get('d', 0) + 1

In [30]: d
Out[30]: {'a': 1, 'b': 2, 'd': 2}

edited Aug 24 '16 at 05:21

answered Sep 09 '12 at 20:46

Ashwini Chaudhary

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@zenpoy is trying to count letters in a text, and your answer is not doing any calculation! – Mp0int Sep 09 '12 at 20:48
I'm not trying to get the value... – zenpoy Sep 09 '12 at 20:48
1

letters[c] = letters.get(c,1) – piokuc Sep 09 '12 at 20:49
3

@piokuc so `letters[c] = letters.get(c,0) + 1` would be better – zenpoy Sep 09 '12 at 20:51
yep, sorry, just wanted to say tbat the Ashwin's answer was not too bad – piokuc Sep 09 '12 at 20:52
@zenpoy poor internet connection didn't allowed me to complete the answer, now done. – Ashwini Chaudhary Sep 09 '12 at 20:52
Anyone has any idea whether `get` is implemented with `try-catch` or with `if-else` and why? – zenpoy Sep 09 '12 at 20:57
1

@zenpoy from `help(dict.get)` --> get(...) `D.get(k[,d]) -> D[k] if k in D, else d`. `d` defaults to `None`. simple ternary operator. – Ashwini Chaudhary Sep 09 '12 at 20:59

score 17 · Accepted Answer · edited May 23 '17 at 12:25

Depending on your python version, you might want to use a defaultdict or a Counter here, as it's the most appropriate.

Now, regarding which alternative is the most pythonic, it depends on the people you're going to ask. On the one hand, exception-based flow control is sometimes frowned upon, as exceptions are supposed to be raised in exceptional situations, and not to be used as conditionals.

On the other hand, however, there are situations in which you would rather use try / except because conditionals would not be practical.

Finally, from a performance standpoint, it depends on whether you expect your key to be there most of the time (An if statement is a bit slower, but an exception is a lot slower when it's raised), and if performance is a concern, you should measure your performance with both implementations before deciding.

All in all, I think that a general rule of thumb would be to use conditionals by default, but use exceptions if they are more practical / make more sense / give you speedups that you really need.

score 10 · Answer 3 · answered Sep 09 '12 at 20:52

10

More pythonic is to use a dedicated tool for the job:

from collections import Counter

with open(fname, 'r') as f:
    letters = Counter(f.read())

Also do note that has_key is deprecated in favor of in.

answered Sep 09 '12 at 20:52

georg

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score 1 · Answer 4 · answered Apr 18 '14 at 19:14

1

see if you can use this trick to handle exception pythonically.

adict = {}
default = None
val = adict.get('dogname', default)
# val will be None rather than raise an exception.

Reference: https://wiki.python.org/moin/KeyError

answered Apr 18 '14 at 19:14

Alan Dong

3,981
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This is Aशwini चhaudhary's answer... – zenpoy Apr 20 '14 at 06:18

Python: try except KeyError vs if has_key()

4 Answers4