Remove Item in Dictionary Based on Value

What is the best way to remove a dictionary item by value in python?

You can use a simple dict comprehension:

myDict = {key:val for key, val in myDict.items() if val != 42}

As such:

>>> {key:val for key, val in myDict.items() if val != 42}
{8: 14, 1: 'egg'}

Remove Item in Dictionary based on Value

Are you trying to remove a single value or all matching values?

If you are trying to remove a single value, how do you define the value you wish to remove?

The reason you don't get a key back when querying on values is because the dictionary could contain multiple keys paired with the specified value.

If you wish to remove all matching instances of the same value, you can do this:

foreach(var item in dic.Where(kvp => kvp.Value == value).ToList())
{
    dic.Remove(item.Key);
}

And if you wish to remove the first matching instance, you can query to find the first item and just remove that:

var item = dic.First(kvp => kvp.Value == value);

dic.Remove(item.Key);

Note: The ToList() call is necessary to copy the values to a new collection. If the call is not made, the loop will be modifying the collection it is iterating over, causing an exception to be thrown on the next attempt to iterate after the first value is removed.

Delete an element from a dictionary

The del statement removes an element:

del d[key]

Note that this mutates the existing dictionary, so the contents of the dictionary changes for anybody else who has a reference to the same instance. To return a new dictionary, make a copy of the dictionary:

def removekey(d, key):
    r = dict(d)
    del r[key]
    return r

The dict() constructor makes a shallow copy. To make a deep copy, see the copy module.

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Note that making a copy for every dict del/assignment/etc. means you're going from constant time to linear time, and also using linear space. For small dicts, this is not a problem. But if you're planning to make lots of copies of large dicts, you probably want a different data structure, like a HAMT (as described in this answer).

How to remove elements from a Python dictionary based on elements in a list?

I would use a dictionary comprehension to map the keys with the values that aren't found within a list:

new_dict = {k: v for k, v in old_dict.items() if v not in the_list} # filter from the list

Removing entries from a dictionary based on values

You can use a dict comprehension:

>>> { k:v for k, v in hand.items() if v }
{'m': 1, 'l': 1}

Or, in pre-2.7 Python, the dict constructor in combination with a generator expression:

>>> dict((k, v) for k, v in hand.iteritems() if v)
{'m': 1, 'l': 1}

How can I remove a key from a Python dictionary?

To delete a key regardless of whether it is in the dictionary, use the two-argument form of dict.pop():

my_dict.pop('key', None)

This will return my_dict[key] if key exists in the dictionary, and None otherwise. If the second parameter is not specified (i.e. my_dict.pop('key')) and key does not exist, a KeyError is raised.

To delete a key that is guaranteed to exist, you can also use

del my_dict['key']

This will raise a KeyError if the key is not in the dictionary.

How to remove elements from lists in dictionaries based on condition in python

You are not accessing the lists correctly. You would want to do:

del cat_map[k][cat_map[k].index(item)]

but you could simplify this check by:

for k,v in cat_map.items():
    if k in v:
        v.remove(k)

How to delete a key and its values from a dictionary based on its position Python

How to remove a key by position

There is not a direct API for indexing dictionaries, but you can list the keys, select a key by position and then remove it from the dictionary.

def remove_key(d, i):
    'Mutate the dictionary to remove i-th key'
    keys = d.keys()
    del d[keys[i]]
    return d

Specific case for "double-ups"

If you data is doubled-up, a dictionary will eliminate the duplicates before you can get to the pairs. So, the data needs to be stored as a list of key/value pairs rather than as a dictionary. That will also allow you to directly delete by position:

>>> receipt = [('beef', 'mint'), ('beef', 'chips'), ('chicken', 'chips')]
>>> del receipt[1]
>>> receipt
[('beef', 'mint'), ('chicken', 'chips')]

Remove a dictionary key that has a certain value

Modifying the original dict:

for k,v in your_dict.items():
    if v == 'DNC':
       del your_dict[k]

or create a new dict using dict comprehension:

your_dict = {k:v for k,v in your_dict.items() if v != 'DNC'}

From the docs on iteritems(),iterkeys() and itervalues():

Using iteritems(), iterkeys() or itervalues() while adding or
deleting entries in the dictionary may raise a RuntimeError or fail
to iterate over all entries.

Same applies to the normal for key in dict: loop.

In Python 3 this is applicable to dict.keys(), dict.values() and dict.items().

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