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03.01 Introducing Pandas Objects

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At the very basic level, Pandas objects can be thought of as enhanced versions of NumPy structured arrays in which the rows and columns are identified with labels rather than simple integer indices. As we will see during the course of this chapter, Pandas provides a host of useful tools, methods, and functionality on top of the basic data structures, but nearly everything that follows will require an understanding of what these structures are. Thus, before we go any further, let's introduce these three fundamental Pandas data structures: the Series, DataFrame, and Index.
We will start our code sessions with the standard NumPy and Pandas imports:
python
import numpy as np
import pandas as pd

The Pandas Series Object

A Pandas Series is a one-dimensional array of indexed data. It can be created from a list or array as follows:
python
data = pd.Series([0.25, 0.5, 0.75, 1.0])
data
0 0.25 1 0.50 2 0.75 3 1.00 dtype: float64
As we see in the output, the Series wraps both a sequence of values and a sequence of indices, which we can access with the values and index attributes. The values are simply a familiar NumPy array:
python
data.values
array([ 0.25, 0.5 , 0.75, 1. ])
The index is an array-like object of type pd.Index, which we'll discuss in more detail momentarily.
python
data.index
RangeIndex(start=0, stop=4, step=1)
Like with a NumPy array, data can be accessed by the associated index via the familiar Python square-bracket notation:
python
data[1]
0.5
python
data[1:3]
1 0.50 2 0.75 dtype: float64
As we will see, though, the Pandas Series is much more general and flexible than the one-dimensional NumPy array that it emulates.

Series as generalized NumPy array

From what we've seen so far, it may look like the Series object is basically interchangeable with a one-dimensional NumPy array. The essential difference is the presence of the index: while the Numpy Array has an implicitly defined integer index used to access the values, the Pandas Series has an explicitly defined index associated with the values.
This explicit index definition gives the Series object additional capabilities. For example, the index need not be an integer, but can consist of values of any desired type. For example, if we wish, we can use strings as an index:
python
data = pd.Series([0.25, 0.5, 0.75, 1.0],
                 index=['a', 'b', 'c', 'd'])
data
a 0.25 b 0.50 c 0.75 d 1.00 dtype: float64
And the item access works as expected:
python
data['b']
0.5
We can even use non-contiguous or non-sequential indices:
python
data = pd.Series([0.25, 0.5, 0.75, 1.0],
                 index=[2, 5, 3, 7])
data
2 0.25 5 0.50 3 0.75 7 1.00 dtype: float64
python
data[5]
0.5

Series as specialized dictionary

In this way, you can think of a Pandas Series a bit like a specialization of a Python dictionary. A dictionary is a structure that maps arbitrary keys to a set of arbitrary values, and a Series is a structure which maps typed keys to a set of typed values. This typing is important: just as the type-specific compiled code behind a NumPy array makes it more efficient than a Python list for certain operations, the type information of a Pandas Series makes it much more efficient than Python dictionaries for certain operations.
The Series-as-dictionary analogy can be made even more clear by constructing a Series object directly from a Python dictionary:
python
population_dict = {'California': 38332521,
                   'Texas': 26448193,
                   'New York': 19651127,
                   'Florida': 19552860,
                   'Illinois': 12882135}
population = pd.Series(population_dict)
population
California 38332521 Florida 19552860 Illinois 12882135 New York 19651127 Texas 26448193 dtype: int64
By default, a Series will be created where the index is drawn from the sorted keys. From here, typical dictionary-style item access can be performed:
python
population['California']
38332521
Unlike a dictionary, though, the Series also supports array-style operations such as slicing:
python
population['California':'Illinois']
California 38332521 Florida 19552860 Illinois 12882135 dtype: int64
We'll discuss some of the quirks of Pandas indexing and slicing in Data Indexing and Selection.

Constructing Series objects

We've already seen a few ways of constructing a Pandas Series from scratch; all of them are some version of the following:
python
>>> pd.Series(data, index=index)
where index is an optional argument, and data can be one of many entities.
For example, data can be a list or NumPy array, in which case index defaults to an integer sequence:
python
pd.Series([2, 4, 6])
0 2 1 4 2 6 dtype: int64
data can be a scalar, which is repeated to fill the specified index:
python
pd.Series(5, index=[100, 200, 300])
100 5 200 5 300 5 dtype: int64
data can be a dictionary, in which index defaults to the sorted dictionary keys:
python
pd.Series({2:'a', 1:'b', 3:'c'})
1 b 2 a 3 c dtype: object
In each case, the index can be explicitly set if a different result is preferred:
python
pd.Series({2:'a', 1:'b', 3:'c'}, index=[3, 2])
3 c 2 a dtype: object
Notice that in this case, the Series is populated only with the explicitly identified keys.

The Pandas DataFrame Object

The next fundamental structure in Pandas is the DataFrame. Like the Series object discussed in the previous section, the DataFrame can be thought of either as a generalization of a NumPy array, or as a specialization of a Python dictionary. We'll now take a look at each of these perspectives.

DataFrame as a generalized NumPy array

If a Series is an analog of a one-dimensional array with flexible indices, a DataFrame is an analog of a two-dimensional array with both flexible row indices and flexible column names. Just as you might think of a two-dimensional array as an ordered sequence of aligned one-dimensional columns, you can think of a DataFrame as a sequence of aligned Series objects. Here, by "aligned" we mean that they share the same index.
To demonstrate this, let's first construct a new Series listing the area of each of the five states discussed in the previous section:
python
area_dict = {'California': 423967, 'Texas': 695662, 'New York': 141297,
             'Florida': 170312, 'Illinois': 149995}
area = pd.Series(area_dict)
area
California 423967 Florida 170312 Illinois 149995 New York 141297 Texas 695662 dtype: int64
Now that we have this along with the population Series from before, we can use a dictionary to construct a single two-dimensional object containing this information:
python
states = pd.DataFrame({'population': population,
                       'area': area})
states
Like the Series object, the DataFrame has an index attribute that gives access to the index labels:
python
states.index
Index(['California', 'Florida', 'Illinois', 'New York', 'Texas'], dtype='object')
Additionally, the DataFrame has a columns attribute, which is an Index object holding the column labels:
python
states.columns
Index(['area', 'population'], dtype='object')
Thus the DataFrame can be thought of as a generalization of a two-dimensional NumPy array, where both the rows and columns have a generalized index for accessing the data.

DataFrame as specialized dictionary

Similarly, we can also think of a DataFrame as a specialization of a dictionary. Where a dictionary maps a key to a value, a DataFrame maps a column name to a Series of column data. For example, asking for the 'area' attribute returns the Series object containing the areas we saw earlier:
python
states['area']
California 423967 Florida 170312 Illinois 149995 New York 141297 Texas 695662 Name: area, dtype: int64
Notice the potential point of confusion here: in a two-dimesnional NumPy array, data[0] will return the first row. For a DataFrame, data['col0'] will return the first column. Because of this, it is probably better to think about DataFrames as generalized dictionaries rather than generalized arrays, though both ways of looking at the situation can be useful. We'll explore more flexible means of indexing DataFrames in Data Indexing and Selection.

Constructing DataFrame objects

A Pandas DataFrame can be constructed in a variety of ways. Here we'll give several examples.
From a single Series object
A DataFrame is a collection of Series objects, and a single-column DataFrame can be constructed from a single Series:
python
pd.DataFrame(population, columns=['population'])
From a list of dicts
Any list of dictionaries can be made into a DataFrame. We'll use a simple list comprehension to create some data:
python
data = [{'a': i, 'b': 2 * i}
        for i in range(3)]
pd.DataFrame(data)
Even if some keys in the dictionary are missing, Pandas will fill them in with NaN (i.e., "not a number") values:
python
pd.DataFrame([{'a': 1, 'b': 2}, {'b': 3, 'c': 4}])
From a dictionary of Series objects
As we saw before, a DataFrame can be constructed from a dictionary of Series objects as well:
python
pd.DataFrame({'population': population,
              'area': area})
From a two-dimensional NumPy array
Given a two-dimensional array of data, we can create a DataFrame with any specified column and index names. If omitted, an integer index will be used for each:
python
pd.DataFrame(np.random.rand(3, 2),
             columns=['foo', 'bar'],
             index=['a', 'b', 'c'])
From a NumPy structured array
We covered structured arrays in Structured Data: NumPy's Structured Arrays. A Pandas DataFrame operates much like a structured array, and can be created directly from one:
python
A = np.zeros(3, dtype=[('A', 'i8'), ('B', 'f8')])
A
array([(0, 0.0), (0, 0.0), (0, 0.0)], dtype=[('A', '<i8'), ('B', '<f8')])
python
pd.DataFrame(A)

The Pandas Index Object

We have seen here that both the Series and DataFrame objects contain an explicit index that lets you reference and modify data. This Index object is an interesting structure in itself, and it can be thought of either as an immutable array or as an ordered set (technically a multi-set, as Index objects may contain repeated values). Those views have some interesting consequences in the operations available on Index objects. As a simple example, let's construct an Index from a list of integers:
python
ind = pd.Index([2, 3, 5, 7, 11])
ind
Int64Index([2, 3, 5, 7, 11], dtype='int64')

Index as immutable array

The Index in many ways operates like an array. For example, we can use standard Python indexing notation to retrieve values or slices:
python
ind[1]
3
python
ind[::2]
Int64Index([2, 5, 11], dtype='int64')
Index objects also have many of the attributes familiar from NumPy arrays:
python
print(ind.size, ind.shape, ind.ndim, ind.dtype)
5 (5,) 1 int64
One difference between Index objects and NumPy arrays is that indices are immutable–that is, they cannot be modified via the normal means:
python
ind[1] = 0

TypeError                                 Traceback (most recent call last)

<ipython-input-34-40e631c82e8a> in <module>()
----> 1 ind[1] = 0


/Users/jakevdp/anaconda/lib/python3.5/site-packages/pandas/indexes/base.py in __setitem__(self, key, value)
   1243 
   1244     def __setitem__(self, key, value):
-> 1245         raise TypeError("Index does not support mutable operations")
   1246 
   1247     def __getitem__(self, key):


TypeError: Index does not support mutable operations
This immutability makes it safer to share indices between multiple DataFrames and arrays, without the potential for side effects from inadvertent index modification.

Index as ordered set

Pandas objects are designed to facilitate operations such as joins across datasets, which depend on many aspects of set arithmetic. The Index object follows many of the conventions used by Python's built-in set data structure, so that unions, intersections, differences, and other combinations can be computed in a familiar way:
python
indA = pd.Index([1, 3, 5, 7, 9])
indB = pd.Index([2, 3, 5, 7, 11])
python
indA & indB  # intersection
Int64Index([3, 5, 7], dtype='int64')
python
indA | indB  # union
Int64Index([1, 2, 3, 5, 7, 9, 11], dtype='int64')
python
indA ^ indB  # symmetric difference

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