for k, v in mydict.iteritems():
    if v is None:
        mydict[k] = ''

In a more general case, e.g. if you were adding or removing keys, it might not be safe to change the structure of the container you're looping on -- so using items to loop on an independent list copy thereof might be prudent -- but assigning a different value at a given existing index does not incur any problem, so, in Python 2.any, it's better to use iteritems.

In Python3 however the code gives AttributeError: 'dict' object has no attribute 'iteritems' error. Use items() instead of iteritems() here.

Refer to this post.

Your suspicions are correct, this is called using comprehension syntax for side-effects, and is normally bad practice.

A better way to do this is with a for loop

for item in x:
    # this also gets rid of your random number list
    item['key3'] = random.random()

You can do it like so, also do not use dict to name the dictionary, its already a reserved name.

In [9]: sample
Out[9]: {'first': 3, 'second': 3}

In [10]: sample = {"first":{"firstInner":1}, "second":{"secondInner":2}}

In [11]: for key,value in sample.items():
    ...:     for k in value.keys():
    ...:         value[k] = 3
    ...: 

In [12]: sample
Out[12]: {'first': {'firstInner': 3}, 'second': {'secondInner': 3}}

The whole idea of dictionaries is that they have unique keys. What you can do is have 'result' as the key and a list as the value, then keep appending to the list.

>>> d = {}
>>> for i in range(0,5):
...      d.setdefault('result', [])
...      d['result'].append(i)
>>> d
{'result': [0, 1, 2, 3, 4]}

The two inner loops seem wrong, you want to traverse the periods and the grades simultaneously, not traversing all the periods for each grade (that's the effect caused by nesting a loop inside another.)

Assuming that there's an equal number of grades and periods the correct way to build a dictionary would be:

final = dict(zip((x.get_text() for x in periods), grades))

The "long" way to do this (as requested in the comments) would be:

final = {}
for period, grade in zip(periods, grades):
    final[period.get_text()] = grade

Also notice that at the end you'll only get a dictionary with the periods/grades of the last class, because you're iterating and creating a new dictionary for each class and discarding the previous dictionaries.

Iterating-and-updating a mapping is pretty gnarly and Python certainly has no support specifically for that, especially when you want to update not just the values but the keys as well: that translates to changing the internal layout of the dictionary on the fly, which affects the order of iteration for instance (you'd pop() each key and reinsert whatever replacement made sense, but then depending on the mapping you might have the issue of trying to replace keys you had not popped yet resulting in inconsistent behaviour).

Technically possible but generally a very bad idea.

The better approach is usually to just create a new dict from the old one e.g.

transformed = dict(
    (
        "type" if key == "sub_type" else key,
        "add" if value == "additive" else value
    )
    for key, value in original.items()
)

Although for the specific case where you want to replace one (key, value) pair if it's present, you can just pop() the offending key then reinsert the replacement e.g.

if val := original.pop("sub_type", None):
    # convert additive to add, leave other types as-is
    if val = "additive":
        val = "add"
    original["type"] = val

Instead of doing assignment you have done an equality comparison.

replace == with =

You can use an iterator to iterate through the values of binary_list instead:

iter_binary_list = iter(binary_list)
for truck in Trucks:
    for day in Days:
        requests[truck][day] = next(iter_binary_list)