This is a cheat sheet for dictionaries in Python. It provides examples for all of the common operations with the standard map type in Python. It covers all the items listed in Essential Interview Code for Maps and Dictionaries.
If you are doing a coding interview in Python, it is recommended to know all of these items, or to keep this list nearby.
It's also a handy list at work, for when some syntax has slipped your mind. Dictionary, and map, syntax changes slightly between languages, thus sometimes it's nice to have a quick reference available.
Basic
# Create a map costs = { 'apple': 3.5, 'banana': 4.2, 'avacado': 9.2, } # Add an element with a key and a value costs['orange'] = 4.1 costs.update({ 'tomato': 3.2, 'lime': 6.9, }) # Check if an element exists by key if 'tomato' in costs: print( "Yummy Red" ) # Get an element by key print( costs['banana'] ) # Get the length of the map print( len( costs ) ) # Check and get value = costs.get( 'avacado' ) if value != None: print( value )
Searching and Removal
# Replace an item by key costs['apple'] = 4.5 # Find an item by value key = list(costs.keys())[list(costs.values()).index(9.2)] # Get by key or default value print( costs.get( 'lemon', 4 ) ) # Delete an element by key del costs['banana'] costs.pop( 'apple' ) # Clear the map costs.clear()
Iteration
# Iterate all key and value pairs in the map for key, value in costs.items(): print( f'{key} = {value}' ) # Iterate the keys of the map for key in costs.keys(): print( key ) # Iterate the values of the map for value in costs.values(): print( value ) # Iterate in sorted order # Use the standard `sorted` function on the pairs, keys, or values for key, value in [(key, costs[key]) for key in sorted(costs.keys())]: print( f'{key} = {value}' )
Full-map operations
We don’t always want to work with the full map, or may wish to isolate certain items.
# Produce a shallow clone of a map shallow_copy = costs.copy() # Map the values of a map to a new map in_cents = {key: math.floor(value * 100) for key, value in costs.items()} # Merge the contents of two maps (new values take precedence) costs.update({ 'coconut': 5.2, 'avacado': 8.7, }) # Filter items by key only_o = {key: value for key, value in costs.items() if key.endswith('o')} # Filter items by value cheap = {key: value for key, value in costs.items() if value < 5}
Mutli-maps
Python does not have a stand multi-map type, but the defaultdict with a list can be used similarly.
# Create a multi-map from collections import defaultdict books = defaultdict(list) # Add an item by key books['jose'].append( 'Red Duck' ) books['jose'].append( 'Blue Duck' ) books['rosie'].append( 'Eight Swans' ) books['greg'].append( 'Greggy' ) # Iterate the items matching a key for book in books['jose']: print( book ) for book in books['nobody']: print( book ) # Remove an item matching a key books['jose'].pop() # Replace all items matching a key books['rosie'] = [ 'Turtle Turtle', 'Alien Invasion' ] # Clear all items with a key books.pop('greg') del books['jose']
