Dictionary: key exist or not

Question

I use django to build my web app. And one of the page sends different files (with different file types). I have 6 file type. And my current version of code (it's really bad - that is why I write this question) is:

    try:
        file = request.FILES[u'file_doc']
    except MultiValueDictKeyError:
        try:
            file = request.FILES[u'file_fb2']
        except MultiValueDictKeyError:
            try:
                file = request.FILES[u'file_pdf']
            except MultiValueDictKeyError:
                try:
                    file = request.FILES[u'file_txt']
                except MultiValueDictKeyError:
                    try:
                        file = request.FILES[u'file_other']
                    except MultiValueDictKeyError:
                        try:
                            file = request.FILES[u'file_chm']
                        except MultiValueDictKeyError:
                            return HttpResponse('bad file type')

Could you advise me - how to improve this bad peace of code.

TIA!

Martijn Pieters · Answer 1 · 2012-08-02T18:23:38.450

6

The in test let's you see if a key is part of a dictionary.

if u'file_doc' in request.FILES:
    file = request.FILES[u'file_doc']

You can simply loop through a set of keys to test for:

for key in (u'file_doc', u'file_fb2', .. ):
    if key in request.FILES:
        file = request.FILES[key]
        break
else:
    return HttpResponse('bad file type')

The else suite will only be executed if the for loop completed without reaching a break statement, i.e. you didn't find a matching key.

Note that you could still use your exception approach in the loop:

for key in (u'file_doc', u'file_fb2', .. ):
    try:
        file = request.FILES[key]
        break
    except MultiValueDictKeyError:
        pass
else:
    return HttpResponse('bad file type')

but I am not so sure that is more readable.

edited Aug 02 '12 at 18:23

answered Aug 02 '12 at 18:17

Martijn Pieters

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Or you could use a `try/except` inside the for loop. tomato vs Tomato -- Or, in the python world, LBYL vs EAFP... (+1) – mgilson Aug 02 '12 at 18:20
Thx! Firs time I afraid of long spaghetti-like try-except code:) – Dmitry Belaventsev Aug 02 '12 at 18:22
@mgilson: and that might be faster too (by a fraction), but I find the `in` test approach a little easier on the eye. Pick according to taste! :-) – Martijn Pieters Aug 02 '12 at 18:24
If the exceptional situation is the common case (and here it is), I prefer LBYL. On the other hand, if the file could be deleted between testing for it's existence and accessing it then EAFP would be the only way to go. – Steven Rumbalski Aug 02 '12 at 18:43

score 3 · Answer 2 · answered Aug 02 '12 at 18:18

3

You could do something like this:

filetypes = [ u'file_doc', u'file_fb2', u'file_pdf' ]

file = None
for ft in filetypes:
  if ft in request.FILES:
    file = request.FILES[ft]
    break

if file is None:
  return HttpResponse('bad file type')

answered Aug 02 '12 at 18:18

larsks

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Wouldn't you want to `break` out of that for-loop when you find the correct filetype? – Sam Mussmann Aug 02 '12 at 18:19
If all three filenames are present, you'll end up with the last match. – Martijn Pieters Aug 02 '12 at 18:20
Thx alot! It's really python-way solution:) – Dmitry Belaventsev Aug 02 '12 at 18:20
@Sam: Yes! I originally had a return there, but realized that didn't fit the way the OP was doing things. Fixed! – larsks Aug 02 '12 at 18:22

score 2 · Answer 3 · edited May 23 '17 at 12:21

2

request.FILES.get(u'file_doc', DEFAULT_VALUE) will return DEFAULT_VALUE if the key does not exist in the dictionary. You can also use the in keyword as mentioned by Martijn edit see also django MultiValueDictKeyError error, how do i deal with it

edited May 23 '17 at 12:21

Community

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answered Aug 02 '12 at 18:18

gsk

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score 1 · Answer 4 · answered Aug 02 '12 at 18:20

1

def GetFile(request):
    file_types = "doc fb2 pdf txt other chm".split()
    for k in file_types:
        if "file_{0}".format(k) in request.FILES:
             return request.FILES["file_{0}".format(k)]

at least thats how I would do it

answered Aug 02 '12 at 18:20

Joran Beasley

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Dictionary: key exist or not

4 Answers4