.. currentmodule:: pandas
.. _dsintro:
************************
Intro to Data Structures
************************
We'll start with a quick, non-comprehensive overview of the fundamental data
structures in pandas to get you started. The fundamental behavior about data
types, indexing, and axis labeling / alignment apply across all of the
objects. To get started, import numpy and load pandas into your namespace:
.. ipython:: python
:suppress:
import numpy as np
from pandas import *
randn = np.random.randn
np.set_printoptions(precision=4, suppress=True)
set_printoptions(precision=4, max_columns=8)
.. ipython:: python
import numpy as np
# will use a lot in examples
randn = np.random.randn
from pandas import *
Here is a basic tenet to keep in mind: **data alignment is intrinsic**. Link
between labels and data will not be broken unless done so explicitly by you.
We'll give a brief intro to the data structures, then consider all of the broad
categories of functionality and methods in separate sections.
.. _basics.series:
Series
------
:class:`Series` is a one-dimensional labeled array (technically a subclass of
ndarray) capable of holding any data type (integers, strings, floating point
numbers, Python objects, etc.). The axis labels are collectively referred to as
the **index**. The basic method to create a Series is to call:
::
>>> s = Series(data, index=index)
Here, ``data`` can be many different things:
- a Python dict
- an ndarray
- a scalar value (like 5)
The passed **index** is a list of axis labels. Thus, this separates into a few
cases depending on what **data is**:
**From ndarray**
If ``data`` is an ndarray, **index** must be the same length as **data**. If no
index is passed, one will be created having values ``[0, ..., len(data) - 1]``.
.. ipython:: python
s = Series(randn(5), index=['a', 'b', 'c', 'd', 'e'])
s
s.index
Series(randn(5))
.. note::
The values in the index must be unique. If they are not, an exception will
**not** be raised immediately, but attempting any operation involving the
index will later result in an exception. In other words, the Index object
containing the labels "lazily" checks whether the values are unique. The
reason for being lazy is nearly all performance-based (there are many
instances in computations, like parts of GroupBy, where the index is not
used).
**From dict**
If ``data`` is a dict, if **index** is passed the values in data corresponding
to the labels in the index will be pulled out. Otherwise, an index will be
constructed from the sorted keys of the dict, if possible.
.. ipython:: python
d = {'a' : 0., 'b' : 1., 'c' : 2.}
Series(d)
Series(d, index=['b', 'c', 'd', 'a'])
.. note::
NaN (not a number) is the standard missing data marker used in pandas
**From scalar value** If ``data`` is a scalar value, an index must be
provided. The value will be repeated to match the length of **index**
.. ipython:: python
Series(5., index=['a', 'b', 'c', 'd', 'e'])
Series is ndarray-like
~~~~~~~~~~~~~~~~~~~~~~
As a subclass of ndarray, Series is a valid argument to most NumPy functions
and behaves similarly to a NumPy array. However, things like slicing also slice
the index.
.. ipython :: python
s[0]
s[:3]
s[s > s.median()]
s[[4, 3, 1]]
np.exp(s)
We will address array-based indexing in a separate :ref:`section <indexing>`.
Series is dict-like
~~~~~~~~~~~~~~~~~~~
A Series is alike a fixed-size dict in that you can get and set values by index
label:
.. ipython :: python
s['a']
s['e'] = 12.
s
'e' in s
'f' in s
If a label is not contained, an exception
.. code-block:: python
>>> s['f']
KeyError: 'f'
>>> s.get('f')
nan
Vectorized operations and label alignment with Series
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When doing data analysis, as with raw NumPy arrays looping through Series
value-by-value is usually not necessary. Series can be also be passed into most
NumPy methods expecting an ndarray.
.. ipython:: python
s + s
s * 2
np.exp(s)
A key difference between Series and ndarray is that operations between Series
automatically align the data based on label. Thus, you can write computations
without giving consideration to whether the Series involved have the same
labels.
.. ipython:: python
s[1:] + s[:-1]
The result of an operation between unaligned Series will have the **union** of
the indexes involved. If a label is not found in one Series or the other, the
result will be marked as missing (NaN). Being able to write code without doing
any explicit data alignment grants immense freedom and flexibility in
interactive data analysis and research. The integrated data alignment features
of the pandas data structures set pandas apart from the majority of related
tools for working with labeled data.
.. note::
In general, we chose to make the default result of operations between
differently indexed objects yield the **union** of the indexes in order to
avoid loss of information. Having an index label, though the data is
missing, is typically important information as part of a computation. You
of course have the option of dropping labels with missing data via the
**dropna** function.
Name attribute
~~~~~~~~~~~~~~
.. _dsintro.name_attribute:
Series can also have a ``name`` attribute:
.. ipython:: python
s = Series(np.random.randn(5), name='something')
s
s.name
The Series ``name`` will be assigned automatically in many cases, in particular
when taking 1D slices of DataFrame as you will see below.
.. _basics.dataframe:
DataFrame
---------
**DataFrame** is a 2-dimensional labeled data structure with columns of
potentially different types. You can think of it like a spreadsheet or SQL
table, or a dict of Series objects. It is generally the most commonly used
pandas object. Like Series, DataFrame accepts many different kinds of input:
- Dict of 1D ndarrays, lists, dicts, or Series
- 2-D numpy.ndarray
- `Structured or record
<http://docs.scipy.org/doc/numpy/user/basics.rec.html>`__ ndarray
- A ``Series``
- Another ``DataFrame``
Along with the data, you can optionally pass **index** (row labels) and
**columns** (column labels) arguments. If you pass an index and / or columns,
you are guaranteeing the index and / or columns of the resulting
DataFrame. Thus, a dict of Series plus a specific index will discard all data
not matching up to the passed index.
If axis labels are not passed, they will be constructed from the input data
based on common sense rules.
From dict of Series or dicts
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The result **index** will be the **union** of the indexes of the various
Series. If there are any nested dicts, these will be first converted to
Series. If no columns are passed, the columns will be the sorted list of dict
keys.
.. ipython:: python
d = {'one' : Series([1., 2., 3.], index=['a', 'b', 'c']),
'two' : Series([1., 2., 3., 4.], index=['a', 'b', 'c', 'd'])}
df = DataFrame(d)
df
DataFrame(d, index=['d', 'b', 'a'])
DataFrame(d, index=['d', 'b', 'a'], columns=['two', 'three'])
The row and column labels can be accessed respectively by accessing the
**index** and **columns** attributes:
.. note::
When a particular set of columns is passed along with a dict of data, the
passed columns override the keys in the dict.
.. ipython:: python
df.index
df.columns
From dict of ndarrays / lists
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The ndarrays must all be the same length. If an index is passed, it must
clearly also be the same length as the arrays. If no index is passed, the
result will be ``range(n)``, where ``n`` is the array length.
.. ipython:: python
d = {'one' : [1., 2., 3., 4.],
'two' : [4., 3., 2., 1.]}
DataFrame(d)
DataFrame(d, index=['a', 'b', 'c', 'd'])
From structured or record array
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
This case is handled identically to a dict of arrays.
.. ipython:: python
data = np.zeros((2,),dtype=[('A', 'i4'),('B', 'f4'),('C', 'a10')])
data[:] = [(1,2.,'Hello'),(2,3.,"World")]
DataFrame(data)
DataFrame(data, index=['first', 'second'])
DataFrame(data, columns=['C', 'A', 'B'])
.. note::
DataFrame is not intended to work exactly like a 2-dimensional NumPy
ndarray.
.. _basics.dataframe.from_list_of_dicts:
From a list of dicts
~~~~~~~~~~~~~~~~~~~~
.. ipython:: python
data2 = [{'a': 1, 'b': 2}, {'a': 5, 'b': 10, 'c': 20}]
DataFrame(data2)
DataFrame(data2, index=['first', 'second'])
DataFrame(data2, columns=['a', 'b'])
.. _basics.dataframe.from_series:
From a Series
~~~~~~~~~~~~~
The result will be a DataFrame with the same index as the input Series, and
with one column whose name is the original name of the Series (only if no other
column name provided).
**Missing Data**
Much more will be said on this topic in the :ref:`Missing data <missing_data>`
section. To construct a DataFrame with missing data, use ``np.nan`` for those
values which are missing. Alternatively, you may pass a ``numpy.MaskedArray``
as the data argument to the DataFrame constructor, and its masked entries will
be considered missing.
Alternate Constructors
~~~~~~~~~~~~~~~~~~~~~~
.. _basics.dataframe.from_dict:
**DataFrame.from_dict**
``DataFrame.from_dict`` takes a dict of dicts or a dict of array-like sequences
and returns a DataFrame. It operates like the ``DataFrame`` constructor except
for the ``orient`` parameter which is ``'columns'`` by default, but which can be
set to ``'index'`` in order to use the dict keys as row labels.
.. _basics.dataframe.from_records:
**DataFrame.from_records**
``DataFrame.from_records`` takes a list of tuples or an ndarray with structured
dtype. Works analogously to the normal ``DataFrame`` constructor, except that
index maybe be a specific field of the structured dtype to use as the index.
For example:
.. ipython:: python
data
DataFrame.from_records(data, index='C')
.. _basics.dataframe.from_items:
**DataFrame.from_items**
``DataFrame.from_items`` works analogously to the form of the ``dict``
constructor that takes a sequence of ``(key, value)`` pairs, where the keys are
column (or row, in the case of ``orient='index'``) names, and the value are the
column values (or row values). This can be useful for constructing a DataFrame
with the columns in a particular order without having to pass an explicit list
of columns:
.. ipython:: python
DataFrame.from_items([('A', [1, 2, 3]), ('B', [4, 5, 6])])
If you pass ``orient='index'``, the keys will be the row labels. But in this
case you must also pass the desired column names:
.. ipython:: python
DataFrame.from_items([('A', [1, 2, 3]), ('B', [4, 5, 6])],
orient='index', columns=['one', 'two', 'three'])
Column selection, addition, deletion
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
You can treat a DataFrame semantically like a dict of like-indexed Series
objects. Getting, setting, and deleting columns works with the same syntax as
the analogous dict operations:
.. ipython:: python
df['one']
df['three'] = df['one'] * df['two']
df['flag'] = df['one'] > 2
df
Columns can be deleted or popped like with a dict:
.. ipython:: python
del df['two']
three = df.pop('three')
df
When inserting a scalar value, it will naturally be propagated to fill the
column:
.. ipython:: python
df['foo'] = 'bar'
df
When inserting a Series that does not have the same index as the DataFrame, it
will be conformed to the DataFrame's index:
.. ipython:: python
df['one_trunc'] = df['one'][:2]
df
You can insert raw ndarrays but their length must match the length of the
DataFrame's index.
By default, columns get inserted at the end. The ``insert`` function is
available to insert at a particular location in the columns:
.. ipython:: python
df.insert(1, 'bar', df['one'])
df
Indexing / Selection
~~~~~~~~~~~~~~~~~~~~
The basics of indexing are as follows:
.. csv-table::
:header: "Operation", "Syntax", "Result"
:widths: 30, 20, 10
Select column, ``df[col]``, Series
Select row by label, ``df.xs(label)`` or ``df.ix[label]``, Series
Select row by location (int), ``df.ix[loc]``, Series
Slice rows, ``df[5:10]``, DataFrame
Select rows by boolean vector, ``df[bool_vec]``, DataFrame
Row selection, for example, returns a Series whose index is the columns of the
DataFrame:
.. ipython:: python
df.xs('b')
df.ix[2]
Note if a DataFrame contains columns of multiple dtypes, the dtype of the row
will be chosen to accommodate all of the data types (dtype=object is the most
general).
For a more exhaustive treatment of more sophisticated label-based indexing and
slicing, see the :ref:`section on indexing <indexing>`. We will address the
fundamentals of reindexing / conforming to new sets of lables in the
:ref:`section on reindexing <basics.reindexing>`.
Data alignment and arithmetic
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Data alignment between DataFrame objects automatically align on **both the
columns and the index (row labels)**. Again, the resulting object will have the
union of the column and row labels.
.. ipython:: python
df = DataFrame(randn(10, 4), columns=['A', 'B', 'C', 'D'])
df2 = DataFrame(randn(7, 3), columns=['A', 'B', 'C'])
df + df2
When doing an operation between DataFrame and Series, the default behavior is
to align the Series **index** on the DataFrame **columns**, thus `broadcasting
<http://docs.scipy.org/doc/numpy/user/basics.broadcasting.html>`__
row-wise. For example:
.. ipython:: python
df - df.ix[0]
In the special case of working with time series data, if the Series is a
TimeSeries (which it will be automatically if the index contains datetime
objects), and the DataFrame index also contains dates, the broadcasting will be
column-wise:
.. ipython:: python
index = DateRange('1/1/2000', periods=8)
df = DataFrame(randn(8, 3), index=index,
columns=['A', 'B', 'C'])
df
type(df['A'])
df - df['A']
Technical purity aside, this case is so common in practice that supporting the
special case is preferable to the alternative of forcing the user to transpose
and do column-based alignment like so:
.. ipython:: python
(df.T - df['A']).T
For explicit control over the matching and broadcasting behavior, see the
section on :ref:`flexible binary operations <basics.binop>`.
Operations with scalars are just as you would expect:
.. ipython:: python
df * 5 + 2
1 / df
df ** 4
.. _dsintro.boolean:
Boolean operators work as well:
.. ipython:: python
df1 = DataFrame({'a' : [1, 0, 1], 'b' : [0, 1, 1] }, dtype=bool)
df2 = DataFrame({'a' : [0, 1, 1], 'b' : [1, 1, 0] }, dtype=bool)
df1 & df2
df1 | df2
df1 ^ df2
-df1
Transposing
~~~~~~~~~~~
To transpose, access the ``T`` attribute (also the ``transpose`` function),
similar to an ndarray:
.. ipython:: python
# only show the first 5 rows
df[:5].T
DataFrame interoperability with NumPy functions
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.. _dsintro.numpy_interop:
Elementwise NumPy ufuncs (log, exp, sqrt, ...) and various other NumPy functions
can be used with no issues on DataFrame, assuming the data within are numeric:
.. ipython:: python
np.exp(df)
np.asarray(df)
The dot method on DataFrame implements matrix multiplication:
.. ipython:: python
df.T.dot(df)
Similarly, the dot method on Series implements dot product:
.. ipython:: python
s1 = Series(np.arange(5,10))
s1.dot(s1)
DataFrame is not intended to be a drop-in replacement for ndarray as its
indexing semantics are quite different in places from a matrix.
Console display
~~~~~~~~~~~~~~~
For very large DataFrame objects, only a summary will be printed to the console
(here I am reading a CSV version of the **baseball** dataset from the **plyr**
R package):
.. ipython:: python
:suppress:
# force a summary to be printed
set_printoptions(max_rows=5)
.. ipython:: python
baseball = read_csv('data/baseball.csv')
print baseball
.. ipython:: python
:suppress:
# restore GlobalPrintConfig
reset_printoptions()
However, using ``to_string`` will return a string representation of the
DataFrame in tabular form, though it won't always fit the console width:
.. ipython:: python
print baseball.ix[-20:, :12].to_string()
DataFrame column types
~~~~~~~~~~~~~~~~~~~~~~
.. _dsintro.column_types:
The four main types stored in pandas objects are float, int, boolean, and
object. A convenient ``dtypes`` attribute return a Series with the data type of
each column:
.. ipython:: python
baseball.dtypes
The related method ``get_dtype_counts`` will return the number of columns of
each type:
.. ipython:: python
baseball.get_dtype_counts()
DataFrame column attribute access and IPython completion
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If a DataFrame column label is a valid Python variable name, the column can be
accessed like attributes:
.. ipython:: python
df = DataFrame({'foo1' : np.random.randn(5),
'foo2' : np.random.randn(5)})
df
df.foo1
The columns are also connected to the `IPython <http://ipython.org>`__
completion mechanism so they can be tab-completed:
.. code-block:: ipython
In [5]: df.fo<TAB>
df.foo1 df.foo2
.. _basics.panel:
Panel
-----
Panel is a somewhat less-used, but still important container for 3-dimensional
data. The term `panel data <http://en.wikipedia.org/wiki/Panel_data>`__ is
derived from econometrics and is partially responsible for the name pandas:
pan(el)-da(ta)-s. The names for the 3 axes are intended to give some semantic
meaning to describing operations involving panel data and, in particular,
econometric analysis of panel data. However, for the strict purposes of slicing
and dicing a collection of DataFrame objects, you may find the axis names
slightly arbitrary:
- **items**: axis 0, each item corresponds to a DataFrame contained inside
- **major_axis**: axis 1, it is the **index** (rows) of each of the
DataFrames
- **minor_axis**: axis 2, it is the **columns** of each of the DataFrames
Construction of Panels works about like you would expect:
From 3D ndarray with optional axis labels
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.. ipython:: python
wp = Panel(randn(2, 5, 4), items=['Item1', 'Item2'],
major_axis=DateRange('1/1/2000', periods=5),
minor_axis=['A', 'B', 'C', 'D'])
wp
From dict of DataFrame objects
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.. ipython:: python
data = {'Item1' : DataFrame(randn(4, 3)),
'Item2' : DataFrame(randn(4, 2))}
Panel(data)
Note that the values in the dict need only be **convertible to
DataFrame**. Thus, they can be any of the other valid inputs to DataFrame as
per above.
One helpful factory method is ``Panel.from_dict``, which takes a
dictionary of DataFrames as above, and the following named parameters:
.. csv-table::
:header: "Parameter", "Default", "Description"
:widths: 10, 10, 40
intersect, ``False``, drops elements whose indices do not align
orient, ``items``, use ``minor`` to use DataFrames' columns as panel items
For example, compare to the construction above:
.. ipython:: python
Panel.from_dict(data, orient='minor')
Orient is especially useful for mixed-type DataFrames. If you pass a dict of
DataFrame objects with mixed-type columns, all of the data will get upcasted to
``dtype=object`` unless you pass ``orient='minor'``:
.. ipython:: python
df = DataFrame({'a': ['foo', 'bar', 'baz'],
'b': np.random.randn(3)})
df
data = {'item1': df, 'item2': df}
panel = Panel.from_dict(data, orient='minor')
panel['a']
panel['b']
panel['b'].dtypes
.. note::
Unfortunately Panel, being less commonly used than Series and DataFrame,
has been slightly neglected feature-wise. A number of methods and options
available in DataFrame are not available in Panel. This will get worked
on, of course, in future releases. And faster if you join me in working on
the codebase.
.. _dsintro.to_panel:
From DataFrame using ``to_panel`` method
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
This method was introduced in v0.7 to replace ``LongPanel.to_long``, and converts
a DataFrame with a two-level index to a Panel.
.. ipython:: python
midx = MultiIndex(levels=[['one', 'two'], ['x','y']], labels=[[1,1,0,0],[1,0,1,0]])
df = DataFrame({'A' : [1, 2, 3, 4], 'B': [5, 6, 7, 8]}, index=midx)
df.to_panel()
.. _dsintro.panel_item_selection:
Item selection / addition / deletion
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Similar to DataFrame functioning as a dict of Series, Panel is like a dict
of DataFrames:
.. ipython:: python
wp['Item1']
wp['Item3'] = wp['Item1'] / wp['Item2']
The API for insertion and deletion is the same as for DataFrame. And as with
DataFrame, if the item is a valid python identifier, you can access it as an
attribute and tab-complete it in IPython.
Indexing / Selection
~~~~~~~~~~~~~~~~~~~~
.. csv-table::
:header: "Operation", "Syntax", "Result"
:widths: 30, 20, 10
Select item, ``wp[item]``, DataFrame
Get slice at major_axis label, ``wp.major_xs(val)``, DataFrame
Get slice at minor_axis label, ``wp.minor_xs(val)``, DataFrame
For example, using the earlier example data, we could do:
.. ipython:: python
wp['Item1']
wp.major_xs(wp.major_axis[2])
wp.minor_axis
wp.minor_xs('C')
Conversion to DataFrame
~~~~~~~~~~~~~~~~~~~~~~~
A Panel can be represented in 2D form as a hierarchically indexed
DataFrame. See the section :ref:`hierarchical indexing <indexing.hierarchical>`
for more on this. To convert a Panel to a DataFrame, use the ``to_frame``
method:
.. ipython:: python
panel = Panel(np.random.randn(3, 5, 4), items=['one', 'two', 'three'],
major_axis=DateRange('1/1/2000', periods=5),
minor_axis=['a', 'b', 'c', 'd'])
panel.to_frame()