Frequently Asked Questions (FAQ)

Adding Features to your pandas Installation

pandas is a powerful tool and already has a plethora of data manipulation operations implemented, most of them are very fast as well. It’s very possible however that certain functionality that would make your life easier is missing. In that case you have several options:

  1. Open an issue on Github , explain your need and the sort of functionality you would like to see implemented.

  2. Fork the repo, Implement the functionality yourself and open a PR on Github.

  3. Write a method that performs the operation you are interested in and Monkey-patch the pandas class as part of your IPython profile startup or PYTHONSTARTUP file.

    For example, here is an example of adding an just_foo_cols() method to the dataframe class:

import pandas as pd
def just_foo_cols(self):
    """Get a list of column names containing the string 'foo'

    return [x for x in self.columns if 'foo' in x]

pd.DataFrame.just_foo_cols = just_foo_cols # monkey-patch the DataFrame class
df = pd.DataFrame([list(range(4))], columns=["A","foo","foozball","bar"])
del pd.DataFrame.just_foo_cols # you can also remove the new method

Monkey-patching is usually frowned upon because it makes your code less portable and can cause subtle bugs in some circumstances. Monkey-patching existing methods is usually a bad idea in that respect. When used with proper care, however, it’s a very useful tool to have.

Migrating from scikits.timeseries to pandas >= 0.8.0

Starting with pandas 0.8.0, users of scikits.timeseries should have all of the features that they need to migrate their code to use pandas. Portions of the scikits.timeseries codebase for implementing calendar logic and timespan frequency conversions (but not resampling, that has all been implemented from scratch from the ground up) have been ported to the pandas codebase.

The scikits.timeseries notions of Date and DateArray are responsible for implementing calendar logic:

In [16]: dt = ts.Date('Q', '1984Q3')

# sic
In [17]: dt
Out[17]: <Q-DEC : 1984Q1>

In [18]: dt.asfreq('D', 'start')
Out[18]: <D : 01-Jan-1984>

In [19]: dt.asfreq('D', 'end')
Out[19]: <D : 31-Mar-1984>

In [20]: dt + 3
Out[20]: <Q-DEC : 1984Q4>

Date and DateArray from scikits.timeseries have been reincarnated in pandas Period and PeriodIndex:

In [1]: pnow('D')  #
Out[1]: Period('2014-07-11', 'D')

In [2]: Period(year=2007, month=3, day=15, freq='D')
Out[2]: Period('2007-03-15', 'D')

In [3]: p = Period('1984Q3')

In [4]: p
Out[4]: Period('1984Q3', 'Q-DEC')

In [5]: p.asfreq('D', 'start')
Out[5]: Period('1984-07-01', 'D')

In [6]: p.asfreq('D', 'end')
Out[6]: Period('1984-09-30', 'D')

In [7]: (p + 3).asfreq('T') + 6 * 60 + 30
Out[7]: Period('1985-07-01 06:29', 'T')

In [8]: rng = period_range('1990', '2010', freq='A')

In [9]: rng
<class 'pandas.tseries.period.PeriodIndex'>
[1990, ..., 2010]
Length: 21, Freq: A-DEC

In [10]: rng.asfreq('B', 'end') - 3
<class 'pandas.tseries.period.PeriodIndex'>
[1990-12-26, ..., 2010-12-28]
Length: 21, Freq: B
scikits.timeseries pandas Notes
Date Period A span of time, from yearly through to secondly
DateArray PeriodIndex An array of timespans
convert resample Frequency conversion in scikits.timeseries
convert_to_annual pivot_annual currently supports up to daily frequency, see GH736

PeriodIndex / DateArray properties and functions

The scikits.timeseries DateArray had a number of information properties. Here are the pandas equivalents:

scikits.timeseries pandas Notes
get_steps np.diff(idx.values)  
has_missing_dates not idx.is_full  
is_full idx.is_full  
is_valid idx.is_monotonic and idx.is_unique  
is_chronological is_monotonic  
arr.sort_chronologically() idx.order()  

Frequency conversion

Frequency conversion is implemented using the resample method on TimeSeries and DataFrame objects (multiple time series). resample also works on panels (3D). Here is some code that resamples daily data to montly:

In [11]: rng = period_range('Jan-2000', periods=50, freq='M')

In [12]: data = Series(np.random.randn(50), index=rng)

In [13]: data
2000-01    0.469112
2000-02   -0.282863
2000-03   -1.509059
2000-04   -1.135632
2000-05    1.212112
2003-09   -0.013960
2003-10   -0.362543
2003-11   -0.006154
2003-12   -0.923061
2004-01    0.895717
2004-02    0.805244
Freq: M, Length: 50

In [14]: data.resample('A', how=np.mean)
2000   -0.394510
2001   -0.244628
2002   -0.221633
2003   -0.453773
2004    0.850481
Freq: A-DEC, dtype: float64


Much of the plotting functionality of scikits.timeseries has been ported and adopted to pandas’s data structures. For example:

In [15]: rng = period_range('1987Q2', periods=10, freq='Q-DEC')

In [16]: data = Series(np.random.randn(10), index=rng)

In [17]: plt.figure(); data.plot()
Out[17]: <matplotlib.axes.AxesSubplot at 0xaa39722c>

Converting to and from period format

Use the to_timestamp and to_period instance methods.

Treatment of missing data

Unlike scikits.timeseries, pandas data structures are not based on NumPy’s MaskedArray object. Missing data is represented as NaN in numerical arrays and either as None or NaN in non-numerical arrays. Implementing a version of pandas’s data structures that use MaskedArray is possible but would require the involvement of a dedicated maintainer. Active pandas developers are not interested in this.

Resampling with timestamps and periods

resample has a kind argument which allows you to resample time series with a DatetimeIndex to PeriodIndex:

In [18]: rng = date_range('1/1/2000', periods=200, freq='D')

In [19]: data = Series(np.random.randn(200), index=rng)

In [20]: data[:10]
2000-01-01   -0.076467
2000-01-02   -1.187678
2000-01-03    1.130127
2000-01-04   -1.436737
2000-01-05   -1.413681
2000-01-06    1.607920
2000-01-07    1.024180
2000-01-08    0.569605
2000-01-09    0.875906
2000-01-10   -2.211372
Freq: D, dtype: float64

In [21]: data.index
<class 'pandas.tseries.index.DatetimeIndex'>
[2000-01-01, ..., 2000-07-18]
Length: 200, Freq: D, Timezone: None

In [22]: data.resample('M', kind='period')
2000-01   -0.175775
2000-02    0.094874
2000-03    0.124949
2000-04    0.066215
2000-05   -0.040364
2000-06    0.116263
2000-07   -0.263235
Freq: M, dtype: float64

Similarly, resampling from periods to timestamps is possible with an optional interval ('start' or 'end') convention:

In [23]: rng = period_range('Jan-2000', periods=50, freq='M')

In [24]: data = Series(np.random.randn(50), index=rng)

In [25]: resampled = data.resample('A', kind='timestamp', convention='end')

In [26]: resampled.index
<class 'pandas.tseries.index.DatetimeIndex'>
[2000-12-31, ..., 2004-12-31]
Length: 5, Freq: A-DEC, Timezone: None

Byte-Ordering Issues

Occasionally you may have to deal with data that were created on a machine with a different byte order than the one on which you are running Python. To deal with this issue you should convert the underlying NumPy array to the native system byte order before passing it to Series/DataFrame/Panel constructors using something similar to the following:

In [27]: x = np.array(list(range(10)), '>i4') # big endian

In [28]: newx = x.byteswap().newbyteorder() # force native byteorder

In [29]: s = Series(newx)

See the NumPy documentation on byte order for more details.

Visualizing Data in Qt applications

There is experimental support for visualizing DataFrames in PyQt4 and PySide applications. At the moment you can display and edit the values of the cells in the DataFrame. Qt will take care of displaying just the portion of the DataFrame that is currently visible and the edits will be immediately saved to the underlying DataFrame

To demonstrate this we will create a simple PySide application that will switch between two editable DataFrames. For this will use the DataFrameModel class that handles the access to the DataFrame, and the DataFrameWidget, which is just a thin layer around the QTableView.

import numpy as np
import pandas as pd
from pandas.sandbox.qtpandas import DataFrameModel, DataFrameWidget
from PySide import QtGui, QtCore

# Or if you use PyQt4:
# from PyQt4 import QtGui, QtCore

class MainWidget(QtGui.QWidget):
    def __init__(self, parent=None):
        super(MainWidget, self).__init__(parent)

        # Create two DataFrames
        self.df1 = pd.DataFrame(np.arange(9).reshape(3, 3),
                                columns=['foo', 'bar', 'baz'])
        self.df2 = pd.DataFrame({
                'int': [1, 2, 3],
                'float': [1.5, 2.5, 3.5],
                'string': ['a', 'b', 'c'],
                'nan': [np.nan, np.nan, np.nan]
            }, index=['AAA', 'BBB', 'CCC'],
            columns=['int', 'float', 'string', 'nan'])

        # Create the widget and set the first DataFrame
        self.widget = DataFrameWidget(self.df1)

        # Create the buttons for changing DataFrames
        self.button_first = QtGui.QPushButton('First')
        self.button_second = QtGui.QPushButton('Second')

        # Set the layout
        vbox = QtGui.QVBoxLayout()
        hbox = QtGui.QHBoxLayout()

    def on_first_click(self):
        '''Sets the first DataFrame'''

    def on_second_click(self):
        '''Sets the second DataFrame'''

if __name__ == '__main__':
    import sys

    # Initialize the application
    app = QtGui.QApplication(sys.argv)
    mw = MainWidget()