Iterate over a dictionary by comprehension and get a dictionary

Question

How to iterate over a dictionary by dictionary comprehension to process it.

>>> mime_types={
    '.xbm': 'image/x-xbitmap',
    '.dwg': 'image/vnd.dwg',
    '.fst': 'image/vnd.fst',
    '.tif': 'image/tiff',
    '.gif': 'image/gif',
    '.ras': 'image/x-cmu-raster',
    '.pic': 'image/x-pict',
    '.fh':  'image/x-freehand',
    '.djvu':'image/vnd.djvu',
    '.ppm': 'image/x-portable-pixmap',
    '.fh4': 'image/x-freehand',
    '.cgm': 'image/cgm',
    '.xwd': 'image/x-xwindowdump',
    '.g3':  'image/g3fax',
    '.png': 'image/png',
    '.npx': 'image/vnd.net-fpx',
    '.rlc': 'image/vnd.fujixerox.edmics-rlc',
    '.svgz':'image/svg+xml',
    '.mmr': 'image/vnd.fujixerox.edmics-mmr',
    '.psd': 'image/vnd.adobe.photoshop',
    '.oti': 'application/vnd.oasis.opendocument.image-template',
    '.tiff':'image/tiff',
    '.wbmp':'image/vnd.wap.wbmp'
}

>>> {(key,val) for key, val in mime_types.items() if "image/tiff" == val}

This is returning result like this:

set([('.tiff', 'image/tiff'), ('.tif', 'image/tiff')])

But I'm expecting

('.tif', 'image/tiff')

How can I modify that result to get a dictionary like :

{'.tif': 'image/tiff'}

Solution 1

The expression:

{ value for bar in iterable }

is a set comprehension.

In order to do a dict comprehension, you have to provide Python with a set of key-value pairs separated by ::

{ key: value for bar in iterable }

Solution 2

Replace

{(key,val) for key, val in mime_types.items() if "image/tiff" == val}

with

{key: val for key, val in mime_types.items() if "image/tiff" == val}

Solution 3

You can do dictionary comprehension as @Anubhav Chattoraj suggested.

Or pass a generator expr as an argument to function dict:

In [165]: dict((k, mimes[k]) for k in mimes if mimes[k] == "image/tiff")
Out[165]: {'.tif': 'image/tiff', '.tiff': 'image/tiff'}

Don't mix the two ways up..

Solution 4

you can try something like this

>>> print {k : v for k, v in mime_types.iteritems()}

Another Simple Example

    >>> print {i : chr(65+i) for i in range(4)}
    {0 : 'A', 1 : 'B', 2 : 'C', 3 : 'D'}

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Comments

[thefourtheye]

It would have been better if you have avoided the .items()

[zhangxaochen]

@thefourtheye why? won't you get ValueError: too many values to unpack?

[thefourtheye]

dict((k, mimes[k]) for k in mimes if mimes[k] == image/tiff") You ll be avoiding the intermediatate list generated by .items()

[zhangxaochen]

@thefourtheye I'm using python3 just now... anyway answer updated ;)

[thefourtheye]

Actually this will work consistently in both Python2 and Python3. But, items() will return an iterator in Py3 but a list in Py2.

[Charlie Parker]

just in case anyone else messes up, the .items() call on the dictionary is crucial.