dataframe.drop(col,axis=1) does not drop column from column.levels in multiindex dataframe

[amol-desai]

I have a multiindex dataframe from which I am dropping columns using df.drop(col,axis=1). Then, I am looking through column.levels[0] and doing some operations on all the columns. However, when I try to do this, pandas looks for the removed column since it is not removed from column.levels. Is this a bug? Is there a WAR?

Here is the df I am working with:

           ^GSPC         PF
        Adj Close  Adj Close
Date                             
2013-04-22    1562.50    4023.45
2013-04-23    1578.78    4099.20
2013-04-24    1578.79    4094.70
2013-04-25    1585.16    4124.25
2013-04-26    1582.24    4211.65
2013-04-29    1593.61    4340.75
2013-04-30    1597.57    4467.55
2013-05-01    1582.70    4432.25
2013-05-02    1597.59    4494.95
2013-05-03    1614.42    4539.55
2013-05-06    1617.50    4645.95
2013-05-07    1625.96    4624.65
2013-05-08    1632.69    4677.40
2013-05-09    1626.67    4637.25
2013-05-10    1633.70    4602.40
2013-05-13    1633.77    4618.60
2013-05-14    1650.34    4510.85
2013-05-15    1658.78    4362.00
2013-05-16    1650.47    4418.95
2013-05-17    1667.47    4406.95
2013-05-20    1666.29    4503.50
2013-05-21    1669.16    4471.20

Here is how I am dropping the columns:

    data = data.drop(stock.ticker,axis=1,level=0)

Here is where the issue is:

    print data.columns
    MultiIndex
    [(^GSPC, Adj Close), (PF, Adj Close)]

    print data.columns.labels
    [array([2, 3]), array([0, 0])]

    print data.columns.levels
    [Index([nvda, aapl, ^GSPC, PF], dtype=object), Index([Adj Close], dtype=object)]

Method used to generate DF as requested in the comment:
tickers is a list of stock ticker strings.
stock is an object that has a ticker property among others.
portfolio is an object that is a collection of stocks.

data = getdata.get_history(tickers,dt.today()-relativedelta(months=months))
data = data.drop(['Open','High','Low','Close','Volume'],axis=1)
data = data.unstack(0).swaplevel(0,1,axis=1).sortlevel(0,axis=1)
data['PF','Adj Close'] = np.zeros(len(data))
for stock in portfolio.getStocksInPortfolio():
  data['PF','Adj Close'] += data[stock.ticker,'Adj Close'] * stock.getSharesOwned()
  data = data.drop(stock.ticker,axis=1,level=0)

[cpcloud]

not sure what you mean by WAR here.

[cpcloud]

I can confirm something similar, this might have something to do with the rebinding of data inside the loop.

randint = np.random.randint
columns = MultiIndex.from_tuples([('gspc', 'adj_close'),
                                    ('pf', 'adj_close'),
                                    ('aapl', 'adj_close'),
                                    ('nvda', 'adj_close')])
n = 50
x = np.random.randn(n) * 31.563 + 1621.127
y = np.random.randn(n) * 181.441 + 4442.121
z = np.random.randn(n) * 31.563 + 1621.127
w = np.random.randn(n) * 181.441 + 4442.121
data = DataFrame(np.column_stack((x, y, z, w)), columns=columns)
stocks = {'nvda': randint(100, 1000), 'gspc': randint(100, 1000), 'aapl': randint(100, 1000)}
data['pf', 'adj_close'] = np.zeros(n)
for ticker, nshares in stocks.iteritems():
    data['pf', 'adj_close'] += data[ticker, 'adj_close'] * nshares
    data = data.drop(ticker, axis=1, level=0)

[amol-desai]

WAR = workaround. Unconsciously threw that in. Sorry.

[michaelaye]

That's a long standing issue: #2770
I still don't understand precisely why Wes does not consider it a bug. Something with not what is observed...
I think either the 'levels' member is to be used for something or to be removed, as this way it is just confusing.

[cpcloud]

It seems to me that if you are explicitly dropping levels then the index should be recomputed since that's what is being requested. As a corner case storing a huge multi index while only storing a single column would be inefficient.

[ghost]

re #2770 (comment), this is not a bug (removed label),
That's not what multiindex levels are for.

The comment about wasting memory is relavent, but since the levels should be shared with the
original frame, that's less of a concern. IPython, which most use, is awful about keeping objects
in memory anyway. If you need a consolidate method, open an issue or use jtranter's suggestion.

@amol-desai, using that, you can reconstruct the frame to "squeeze" out unused level factors.
You should be looking through labels rather then levels, though, (depending on what you're tying to do).

[jreback]

closing as not a bug