There are two ways to add one dictionary to another:

Update (modifies orig in place)

orig.update(extra)    # Python 2.7+
orig |= extra         # Python 3.9+

Merge (creates a new dictionary)

# Python 2.7+
dest = collections.ChainMap(orig, extra)
dest = {k: v for d in (orig, extra) for (k, v) in d.items()}

# Python 3
dest = {**orig, **extra}          
dest = {**orig, 'D': 4, 'E': 5}

# Python 3.9+ 
dest = orig | extra

Caveats

Note that these operations are noncommutative. In all cases, the latter is the winner. E.g.

orig  = {'A': 1, 'B': 2}
extra = {'A': 3, 'C': 3}

dest = orig | extra
# dest = {'A': 3, 'B': 2, 'C': 3}

dest = extra | orig
# dest = {'A': 1, 'B': 2, 'C': 3}

It is also important to note that only from Python 3.7 (and CPython 3.6) dicts are ordered. So, in previous versions, the order of the items in the dictionary may vary.

I feel like consolidating info about Python dictionaries:

Creating an empty dictionary

data = {}
# OR
data = dict()

Creating a dictionary with initial values

data = {'a': 1, 'b': 2, 'c': 3}
# OR
data = dict(a=1, b=2, c=3)
# OR
data = {k: v for k, v in (('a', 1), ('b',2), ('c',3))}

Inserting/Updating a single value

data['a'] = 1  # Updates if 'a' exists, else adds 'a'
# OR
data.update({'a': 1})
# OR
data.update(dict(a=1))
# OR
data.update(a=1)

Inserting/Updating multiple values

data.update({'c':3,'d':4})  # Updates 'c' and adds 'd'

Python 3.9+:

The update operator |= now works for dictionaries:

data |= {'c':3,'d':4}

Creating a merged dictionary without modifying originals

data3 = {}
data3.update(data)  # Modifies data3, not data
data3.update(data2)  # Modifies data3, not data2

Python 3.5+:

This uses a new feature called dictionary unpacking.

data = {**data1, **data2, **data3}

Python 3.9+:

The merge operator | now works for dictionaries:

data = data1 | {'c':3,'d':4}

Deleting items in dictionary

del data[key]  # Removes specific element in a dictionary
data.pop(key)  # Removes the key & returns the value
data.clear()  # Clears entire dictionary

Check if a key is already in dictionary

key in data

Iterate through pairs in a dictionary

for key in data: # Iterates just through the keys, ignoring the values
for key, value in d.items(): # Iterates through the pairs
for key in d.keys(): # Iterates just through key, ignoring the values
for value in d.values(): # Iterates just through value, ignoring the keys

Create a dictionary from two lists

data = dict(zip(list_with_keys, list_with_values))