与单元素元组混淆

Question

here below the codes i tried to locate the relative maxima and minima of stock prices and make a plot with matplotlib

stockewma=pd.ewma(stock,span=3)
stocka=np.array(stockewma.values)
mx=argrelextrema(stocka,np.greater)
mn=argrelextrema(stocka,np.less)
mxx=[stock.index[i] for i in mx]
mxy=[stock.ix[i] for i in mx]
mnx=[stock.index[i] for i in mn]
mny=[stock.ix[i] for i in mn]

the code works alright, but then i found out the argrelextrema actually returns a tuple of SINGLE np.array of a SINGLE list.

why should it wrap the data as clumsy as that?

for that i have to access the index value with mx[0] #cuz it is a single element tuple.

and i don't understand why the following code could work, index with nesting a array of single list?!

mxx=[stock.index[i] for i in mx]
mxy=[stock.ix[i] for i in mx]
mnx=[stock.index[i] for i in mn]
mny=[stock.ix[i] for i in mn]

Answer 1

argrelextrema returns a single element tuple so that the following mxx and mxy statements work. These are list comprehensions

The statement

mxx=[stock.index[i] for i in mx]

returns a list which could be created by the following equivalent code

mxx = []
for i in mx:
    value = stock.index[i]
    mxx.append(value)