For Python 3+:

>>> mydict
{'four': 4, 'three': 3, 'one': 1}

>>> for k in list(mydict.keys()):
...     if mydict[k] == 3:
...         del mydict[k]

>>> mydict
{'four': 4, 'one': 1}

The other answers work fine with Python 2 but raise a RuntimeError for Python 3:

RuntimeError: dictionary changed size during iteration.

This happens because mydict.keys() returns an iterator not a list. As pointed out in comments simply convert mydict.keys() to a list by list(mydict.keys()) and it should work.

For Python 2:

A simple test in the console shows you cannot modify a dictionary while iterating over it:

>>> mydict = {'one': 1, 'two': 2, 'three': 3, 'four': 4}

>>> for k, v in mydict.iteritems():
...    if k == 'two':
...        del mydict[k]

------------------------------------------------------------
Traceback (most recent call last):
  File "<ipython console>", line 1, in <module>
RuntimeError: dictionary changed size during iteration

As stated in delnan's answer, deleting entries causes problems when the iterator tries to move onto the next entry. Instead, use the keys() method to get a list of the keys and work with that:

>>> for k in mydict.keys():
...    if k == 'two':
...        del mydict[k]

>>> mydict
{'four': 4, 'three': 3, 'one': 1}

If you need to delete based on the items value, use the items() method instead:

>>> for k, v in mydict.items():
...     if v == 3:
...         del mydict[k]

>>> mydict
{'four': 4, 'one': 1}

Here is a version modifying your first example, you will need to "copy" your list to iterate with it and deleting at the same time. After you're iterating with the copied list, you can delete from the original list as necessary.

import copy

qr = [{'A': 'B', 'C': '3', 'EE': None, 'P': '343', 'AD': ' ', 'B': ''},
    {'A': 'B', 'C': '3', 'EE': None, 'P': '343', 'AD': ' ', 'B': ''}]

for i, row in enumerate(copy.deepcopy(qr)):
     for key, value in row.items():
         if value in {' ', None, ''}:
             del qr[i][key]

print(qr)

Other than that, usually you want to create a new list than to delete from the original list. A simple list comprehension will do the trick:

qr = [{k:v for k, v in row.items() if v not in {' ', None, ''}} for row in qr]

print(qr) # same result

Output for both:

[{'A': 'B', 'C': '3', 'P': '343'},
 {'A': 'B', 'C': '3', 'P': '343'}]

Due to the fact that Python dictionaries are implemented as hash tables, you shouldn't rely on them having any sort of an order. Key order may change unpredictably (but only after insertion or removal of a key). Thus, it's impossible to predict the next key. Python throws the RuntimeError to be safe, and to prevent people from running into unexpected results.

Python 2's dict.items method returns a copy of key-value pairs, so you can safely iterate over it and delete values you don't need by keys, as @wim suggested in comments. Example:

for k, v in my_dict.items():
    if v < threshold_value:
        del my_dict[k]

However, Python 3's dict.items returns a view object that reflects all changes made to the dictionary. This is the reason the solution above only works in Python 2. You may convert my_dict.items() to list (tuple etc.) to make it Python 3-compatible.

Another way to approach the problem is to select keys you want to delete and then delete them

keys = [k for k, v in my_dict.items() if v < threshold_value]
for x in keys:
    del my_dict[x]

This works in both Python 2 and Python 3.

For Python-3-x the easy way is to convert the dict into a list() in the iteration:

mydict = {'one': 1, 'two': 2, 'three': 3, 'four': 4}

for k, v in list(mydict.items()):
    if k == 'two':
        del(mydict[k])
        continue   
    print(k)