Frequently Asked Questions (FAQ)¶
DataFrame memory usage¶
As of pandas version 0.15.0, the memory usage of a dataframe (including the index) is shown when accessing the info method of a dataframe. A configuration option, display.memory_usage (see Options and Settings), specifies if the dataframe’s memory usage will be displayed when invoking the df.info() method.
For example, the memory usage of the dataframe below is shown when calling df.info():
In [1]: dtypes = ['int64', 'float64', 'datetime64[ns]', 'timedelta64[ns]',
...: 'complex128', 'object', 'bool']
...:
In [2]: n = 5000
In [3]: data = dict([ (t, np.random.randint(100, size=n).astype(t))
...: for t in dtypes])
...:
In [4]: df = pd.DataFrame(data)
In [5]: df['categorical'] = df['object'].astype('category')
In [6]: df.info()
<class 'pandas.core.frame.DataFrame'>
Int64Index: 5000 entries, 0 to 4999
Data columns (total 8 columns):
bool 5000 non-null bool
complex128 5000 non-null complex128
datetime64[ns] 5000 non-null datetime64[ns]
float64 5000 non-null float64
int64 5000 non-null int64
object 5000 non-null object
timedelta64[ns] 5000 non-null timedelta64[ns]
categorical 5000 non-null category
dtypes: bool(1), category(1), complex128(1), datetime64[ns](1), float64(1), int64(1), object(1), timedelta64[ns](1)
memory usage: 303.5+ KB
The + symbol indicates that the true memory usage could be higher, because pandas does not count the memory used by values in columns with dtype=object.
By default the display option is set to True but can be explicitly overridden by passing the memory_usage argument when invoking df.info().
The memory usage of each column can be found by calling the memory_usage method. This returns a Series with an index represented by column names and memory usage of each column shown in bytes. For the dataframe above, the memory usage of each column and the total memory usage of the dataframe can be found with the memory_usage method:
In [7]: df.memory_usage()
Out[7]:
bool 5000
complex128 80000
datetime64[ns] 40000
float64 40000
int64 40000
object 20000
timedelta64[ns] 40000
categorical 5800
dtype: int64
# total memory usage of dataframe
In [8]: df.memory_usage().sum()
Out[8]: 270800
By default the memory usage of the dataframe’s index is not shown in the returned Series, the memory usage of the index can be shown by passing the index=True argument:
In [9]: df.memory_usage(index=True)
Out[9]:
Index 40000
bool 5000
complex128 80000
datetime64[ns] 40000
float64 40000
int64 40000
object 20000
timedelta64[ns] 40000
categorical 5800
dtype: int64
The memory usage displayed by the info method utilizes the memory_usage method to determine the memory usage of a dataframe while also formatting the output in human-readable units (base-2 representation; i.e., 1KB = 1024 bytes).
See also Categorical Memory Usage.
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 [10]: x = np.array(list(range(10)), '>i4') # big endian
In [11]: newx = x.byteswap().newbyteorder() # force native byteorder
In [12]: s = pd.Series(newx)
See the NumPy documentation on byte order for more details.
Visualizing Data in Qt applications¶
Warning
The qt support is deprecated and will be removed in a future version. We refer users to the external package pandas-qt.
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_first.clicked.connect(self.on_first_click)
self.button_second = QtGui.QPushButton('Second')
self.button_second.clicked.connect(self.on_second_click)
# Set the layout
vbox = QtGui.QVBoxLayout()
vbox.addWidget(self.widget)
hbox = QtGui.QHBoxLayout()
hbox.addWidget(self.button_first)
hbox.addWidget(self.button_second)
vbox.addLayout(hbox)
self.setLayout(vbox)
def on_first_click(self):
'''Sets the first DataFrame'''
self.widget.setDataFrame(self.df1)
def on_second_click(self):
'''Sets the second DataFrame'''
self.widget.setDataFrame(self.df2)
if __name__ == '__main__':
import sys
# Initialize the application
app = QtGui.QApplication(sys.argv)
mw = MainWidget()
mw.show()
app.exec_()