Constructing a new dict:

dict_you_want = {key: old_dict[key] for key in your_keys}

Uses dictionary comprehension.

If you use a version which lacks them (ie Python 2.6 and earlier), make it dict((key, old_dict[key]) for ...). It's the same, though uglier.

Note that this, unlike jnnnnn's version, has stable performance (depends only on number of your_keys) for old_dicts of any size. Both in terms of speed and memory. Since this is a generator expression, it processes one item at a time, and it doesn't looks through all items of old_dict.

Removing everything in-place:

unwanted = set(old_dict) - set(your_keys)
for unwanted_key in unwanted: del your_dict[unwanted_key]

You can use the .keys() method:

for key in myDictionary.keys():
   print(key)

You can also use .items() to iterate through both at the same time:

for key, value in myDictionary.items():
   print(key, value)

This is probably what you were looking for.

Note this is for Python 3.7 and forward.

d = {1: 'a', 2: 'b'} # dict is a bad variable name! don't do it! it overrides a built-in!
d_keys = list(d.keys())
first_value = d[d_keys[0]]

You can try something like this:

In [1]: dictlist = [{'Name': 'James', 'city': 'paris','type': 'A' },  
   ...:             {'Name': 'James','city': 'Porto','type': 'B'}, 
   ...:             {'Name': 'Christian','city': 'LA','type': 'A'}]                                                                                                                                         

In [2]: keys = ["Name","type"]                                                                                                                                                                              

In [3]: res = []                                                                                                                                                                                            

In [5]: for dict1 in dictlist: 
   ...:     result = dict((k, dict1[k]) for k in keys if k in dict1) 
   ...:     res.append(result) 
   ...:                                                                                                                                                                                                     

In [6]: res                                                                                                                                                                                                 
Out[6]: 
[{'Name': 'James', 'type': 'A'},
 {'Name': 'James', 'type': 'B'},
 {'Name': 'Christian', 'type': 'A'}]

If you truly want to create a new dictionary with the keys in your list, then you can use a dict comprehension.

a = {"a":1,"b":2,"c":3,"f":4,"d":5}
b = ["b", "c"]

out = {x: a[x] for x in b}

This will fail by raising a KeyError if any of the elements of b are not actually keys in a.

If you are checking each keys value, simply check all values:

if input_a not in dictionary.values():
        print('Sorry, try again')

If you have iterables as values:

if not any(input_a in  ele for ele in dictionary.values()):
        print('Sorry, try again')

I used any as I presume if input_a was equal to 2 and some keys value was equal to [1,2,3] then it should return True.

Your code new = {key: value_keys[key] for key in ['text', 'lang']} is doing the same as this code:

for key in ['text', 'lang']:
   new[key] = value_keys[key]

You are essentially iterating over each key in value_keys and assigning the same key and the same value to a new dictionary called new.

value_keys[key] returns the value of key in the dictionary value_keys.

Answer 1)

So I need someone to clarify why I have to put key: value_keys[key] In this manner to extract the keys from the dictionary.

You do not have to put key: value_keys[key] in that manner to extract the keys from the dictionary. You can also use for loops:

for key in value_keys
   print(key)

or

for key in value_keys.keys()
   print(key)

Python How to iterate through dictionary

Answer 2)

Next, I need to know why I can't select specific keys from this code value_keys.keys().

value_keys.keys() is returning a list of all the keys in the value_keys dictionary, you can select a specific key from the list by iterating through the list of all keys and checking if the key meets a condition (in this example I chose to select the key if it was called 'text'):

for key in value_keys.keys():
   if(key == "text"):
      print(key)

If you just want to check whether or not a key is in a dictionary you can use:

if key in value_keys:
   #code to execute if the key is in the dictionary
else:
   #code to execute if the key is not in the dictionary

Python check if key exists in dictionary

I think you're misunderstanding what filter() does. From the docs:

filter(function, iterable):
Construct an iterator from those elements of iterable for which function returns true.

So when you do

hits = dict(filter(lambda item: 'hit' in item[0], topHitsDict['record301'].items()))

You're essentially doing:

hits = {}
for item in topHitsDict['record301'].items():
    if 'hit' in item[0]:
        hits[item[0]] = item[1]

which gives you only the hit* keys from topHitsDict['record301'].

hits = {'hit1': {'description': 'OBGP-2018-240_Oncorhynchus.clarkii',
  'score': '340',
  'eval': '2e-94'},
 'hit2': {'description': 'OBGP-2017-332_Oncorhynchus.clarkii',
  'score': '340',
  'eval': '2e-94'}}

Instead, what you actually want is the description from those hit* dicts. For this, you can use map, and then convert the iterator to a list.

descriptions = list(map(lambda item: item[1]['description'], hits.items())
# descriptions: ['OBGP-2018-240_Oncorhynchus.clarkii', 'OBGP-2017-332_Oncorhynchus.clarkii']

This is equivalent to:

descriptions = []
for item in hits.items():
    descriptions.append(item[1]['description'])

And if you want to do this for all keys of topHitsDict, you'd have to change it up a bit. Either using a loop:

all_descriptions = []
for recordVal in topHitsDict.values():
    hits = dict(filter(lambda item: 'hit' in item[0], recordVal.items()))
    descriptions = list(map(lambda item: item[1]['description'], hits.items())
    # Add to all_descriptions
    all_descriptions = all_descriptions + descriptions

It's almost always easier to write these out as a loop first. Then you can write them as a list- or dict- comprehension, and then use filter() and map()

all_descriptions = []

for record in topHitsDict.values():
    for hitname, hitval in record.items():
        if "hit" in hitname:
            all_descriptions.append(hitval['description'])

Or as a comprehension:

all_descriptions = [hitval["description"] for record in topHitsDict.values() for hitname, hitval in record.items() if "hit" in hitname]