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efueyo
Asked: 2022-07-12 01:08:23 +0800 CST 2022-07-12 01:08:23 +0800 CST

dictionary of dictionaries

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I show below a script that imports historical quotes of different stocks from Yahoo, using 'pandas_datareader' .

# 1 - Define `tickers` & `company names` for every instrument
stocks      = {'AAPL':'Apple', 'MSFT':'Microsoft', 'AMZN' : 'Amazon',  'GOOG': 'Google', 'FB':'Facebook','NFLX':'Netflix' ,  'NVDA' : 'NVIDIA'}
bonds       = {'HCA' : 'HCA', 'VRTX' :  'VRTX'}
commodities = {'BTC-USD' : 'Bitcoin', 'PA=F' : 'Palladium'}
instruments = {**stocks, **bonds, **commodities}
tickers     = list(instruments.keys())
instruments_data = {}
N = len(tickers)


# 2 - We will look at stock prices over the past years, starting at January 1, 2015
#                               01-01-2015 - 16-04-2020
start = datetime.datetime(2015,1,1)
end   = datetime.datetime(2020,4,16)

# 3 - Let's get instruments data based on the tickers.
# First argument is the series we want, second is the source ("yahoo" for Yahoo! Finance), third is the start date, fourth is the end date
for ticker, instrument in instruments.items():
  print("Loading data series for instrument {} with ticker = {}".format(instruments[ticker], ticker))
  instruments_data[ticker] = web.DataReader(ticker, data_source = 'yahoo', start = start, end = end)

The statement instruments = {**stocks, **bonds, **commodities}, I guess creates a dictionary with three dictionaries. I don't understand what function the two '**' have. I will be grateful if you explain this sentence to me.

python-3.x

2 Answers

  1. Best Answer

    Thanks to this python treats what you give it as a key-value, with this it copies the dictionary with key-value into the new dictionary.

    Having the following dictionaries:

    stocks      = {'AAPL':'Apple', 'MSFT':'Microsoft', 'AMZN' : 'Amazon',  'GOOG': 'Google', 'FB':'Facebook','NFLX':'Netflix' ,  'NVDA' : 'NVIDIA'}
bonds       = {'HCA' : 'HCA', 'VRTX' :  'VRTX'}
commodities = {'BTC-USD' : 'Bitcoin', 'PA=F' : 'Palladium'}

    Example:

    The part of your code:

    instruments = {**stocks, **bonds, **commodities}
print(instruments)

    This shows the following dictionary (All dictionaries in one):

    {'AAPL': 'Apple', 'MSFT': 'Microsoft', 'AMZN': 'Amazon', 'GOOG': 'Google', 'FB': 'Facebook', 'NFLX': 'Netflix', 'NVDA': 'NVIDIA', 'HCA': 'HCA', 'VRTX': 'VRTX', 'BTC-USD': 'Bitcoin', 'PA=F': 'Palladium'}

    However, if you put only an asterisk (*) it copies only the key.

    Examples (Using the dictionaries from before):

    instruments = {*stocks, *bonds, *commodities}
print(instruments)

    Returns the following set (The keys of all dictionaries in a set):

    {'PA=F', 'NVDA', 'AMZN', 'GOOG', 'MSFT', 'BTC-USD', 'VRTX', 'AAPL', 'FB', 'HCA', 'NFLX'}

    And this other example:

    instruments = (*stocks, *bonds, *commodities)
print(instruments)

    Return the following tuple (The keys of all dictionaries in one tuple):

    ('AAPL', 'MSFT', 'AMZN', 'GOOG', 'FB', 'NFLX', 'NVDA', 'HCA', 'VRTX', 'BTC-USD', 'PA=F')

    And finally:

    instruments = [*stocks, *bonds, *commodities]
print(instruments)

    Show the following list (The keys of all the dictionaries in a list):

    ['AAPL', 'MSFT', 'AMZN', 'GOOG', 'FB', 'NFLX', 'NVDA', 'HCA', 'VRTX', 'BTC-USD', 'PA=F']
    • 3

  2. I would like to add that the two asterisks ** mean an arbitrary number of arguments.

    They are useful when a function can take an arbitrary/unknown number of arguments. It turns those variables into a dictionary, effectively, and the function can verify that a certain variable was entered by checking the curly braces.

    def miFunc(**miDict):
    print(miDict)
    if 'pago' in miDict.keys():
        print("Pago procesado.")
    else:
        print("Transacción cancelada: Fondos insuficientes.")
    
miFunc(naranjas=3,manzanas=10)
# Resultado:
# {'naranjas': 3, 'manzanas': 10}
# Transacción cancelada: Fondos insuficientes.

miFunc(naranjas=3,manzanas=10,pago=3.5)
# Resultado:
# {'naranjas': 3, 'manzanas': 10, 'pago': 3.5}
# Pago procesado.

    Functions that take an arbitrary number of arguments are useful when you are processing texts of arbitrary size/length. Each word is being tagged with semantic or grammatical information, for example, to process the sentiment of a comment.

    It is worth noting that already in python3.9 this statement

    instruments = {**stocks, **bonds, **commodities}

    can be reformulated:

    instruments = stocks | bonds | commodities

    I see it as more readable, what programming should be.

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