A Series is like an array: a 1-D list of homogenously-typed items.

In [1]:
import pandas as pd
a = pd.Series([1, 2, 3])
a

Out[1]:
0    1
1    2
2    3
In [2]:
a.dtype

Out[2]:
dtype('int64')

Here's a Series() of floating point numbers:

In [3]:
b = pd.Series([1, 2.3, 3])
b

Out[3]:
0    1.0
1    2.3
2    3.0
In [4]:
b.dtype

Out[4]:
dtype('float64')

Of course if you mix things up, everything in Python is an object in the end.

In [5]:
c = pd.Series(['a', None, 5])
c

Out[5]:
0       a
1    None
2       5
In [6]:
c.dtype

Out[6]:
dtype('object')

# Broadcasting operations across a Series¶

You can apply conditional expressions to a Series, and it will return another Series with the result of that expression applied to each value. NumPy calls this "broadcasting".

In [7]:
a

Out[7]:
0    1
1    2
2    3
In [8]:
a > 1

Out[8]:
0    False
1     True
2     True
In [9]:
a == 1

Out[9]:
0     True
1    False
2    False

It's also easy to broadcast your own callable by using Series.map():

In [10]:
a.map(lambda x: x % 2 == 0)

Out[10]:
0    False
1     True
2    False

# DataFrames¶

A DataFrame is essentially a set of Series objects (as columns) with a shared index (the row labels).

In [11]:
d = pd.DataFrame(
[
[1, 2.3, 'three'],
[4, 5, 6],
[7, 8, 9],
[10, 11, 12]],
columns=['Integers', 'Floats', 'Objects'],
index=[1, 2, 3, 4])
d

Out[11]:
Integers Floats Objects
1 1 2.3 three
2 4 5.0 6
3 7 8.0 9
4 10 11.0 12
In [12]:
d.dtypes

Out[12]:
Integers      int64
Floats      float64
Objects      object

# Selecting data¶

Selecting by column by using a key lookup:

In [13]:
d['Floats']

Out[13]:
1     2.3
2     5.0
3     8.0
4    11.0
Name: Floats

You can look up two columns by indexing using a list of columns:

In [14]:
d[['Integers', 'Objects']]

Out[14]:
Integers Objects
1 1 three
2 4 6
3 7 9
4 10 12

You can select a range of rows using list slices. Note that this refers to the rows as if they were in a Python list()!

In [15]:
d[2:]

Out[15]:
Integers Floats Objects
3 7 8 9
4 10 11 12

You can also avoid the magic and just use DataFrame.xs() to access the rows by their indexed name:

In [16]:
d.xs(3, axis=0)

Out[16]:
Integers    7
Floats      8
Objects     9
Name: 3

Or specifying column names:

In [17]:
d.xs('Floats', axis=1)

Out[17]:
1     2.3
2     5.0
3     8.0
4    11.0
Name: Floats

Row indexing can also be done using a mask:

In [18]:
mask = [True, False, True, False]

Out[18]:
Integers Floats Objects
1 1 2.3 three
3 7 8.0 9

Combined with conditional expression broadcasting, and you can get some really interesting results:

In [19]:
where_Integers_is_gt_4 = d['Integers'] > 4
d[where_Integers_is_gt_4]

Out[19]:
Integers Floats Objects
3 7 8 9
4 10 11 12
In [20]:
d[np.invert(where_Integers_is_gt_4)]

Out[20]:
Integers Floats Objects
1 1 2.3 three
2 4 5.0 6

To get a subset of the rows based on the index value:

In [21]:
d[d.index > 2]

Out[21]:
Integers Floats Objects
3 7 8 9
4 10 11 12
In [22]:
imshow(imread('masking.png'), cmap='gray')

Out[22]:
<matplotlib.image.AxesImage at 0x10521a3d0>
In [22]: