# Introduction to Pandas¶

Tyler Caraza-Harter

Many datasets you'll encounter are tabular; in other words, the data can be organized with tables and columns. We've seen how to organize this data with lists of lists, but this is cumbersome. Now we'll learn Pandas, a Python module built specifically for tabular data. If you become comfortable with Pandas, you'll likely start preferring it over Excel for analyzing tables.

We need to install Pandas:

pip install pandas

Then import it:

In [1]:
import pandas as pd


The as pd expression may be new for you. This just gives the pandas module a new name in our code, so we can type things like pd.some_function() to call a function named some_function rather than type out pandas.some_function. You could also have just used import pandas or even given it another name with an import like import pandas as silly_bear, but we recommend you import as "pd", because most pandas users do so by convention.

We'll also be using two new data types (Series and DataFrame) from Pandas very often, so let's import these directly so we don't even need to prefix their use with pd..

In [2]:
from pandas import Series, DataFrame


# Pandas Series¶

Pandas tables are built as collections of Pandas Series. A Series is a sophisticated data structure that combines many of the features of both Python lists and dicts.

You can think of a Series as a dictionary where the values are ordered and, in addition to having a key, are labeled with integer positions (0, 1, 2, etc).

## Careful with the Vocabulary!¶

The terms we'll use when talking about Series are not very consistent with the terms we used for lists and dicts. I would happily invent new terminology and teach it to you, but I think it's better to teach you the vocubalary you'll see used outside of CS 301.

A pandas integer position refers to a 0, 1, 2, etc. label, and is equivalent to a list's index.

A pandas index refers to the programmer-chosen label (which could be a string, int, etc) on a value, and is equivalent to a dict's key.

After all the fuss we've made about disntinguishing a list's index from a dict's key, it's fair for you to be unhappy about what "index" means in the context of pandas.

## Series vs. Dictionary¶

It is easy to convert a dict to a Series:

In [93]:
d = {"one": 7, "two": 8, "three": 9}
d

Out[93]:
{'one': 7, 'two': 8, 'three': 9}
In [94]:
# dict to Series
s = Series(d)
s

Out[94]:
one      7
two      8
three    9
dtype: int64

In this case, the values are 7, 8, and 9, and the corresponding indexes are "one", "two", and "three".

dtype stands for data type. In this case, it means that the series contains integers, each of which require 64 bits of memory (this detail is not important for us). Although you could create a Series containing different types of data (as with lists), we'll avoid doing so because working with Series of one type will often be more convenient. You may mix values of different types in a Series (just like we regularly do with lists), but this is discouraged.

A Series can be expected to keep the same order (unless we intentionally change it), unlike a dict.

Let's lookup a value using some indexes, using .loc[???]:

In [95]:
print(s.loc["one"])
print(s.loc["three"])
print(s.loc["two"])

7
9
8


Instead of .loc we can use .iloc to do lookup by integer position:

In [96]:
print(s.iloc[0])
print(s.iloc[2])

7
9


If you don't use .loc or .iloc, pandas tries to figure out which one you mean:

In [97]:
print(s["one"])
print(s[0])

7
7


Of course, this can create ambiguities (should s[0] produce "A" or "C"?).

In [101]:
s = Series({2: "A", 1: "B", 0: "C"})
s

Out[101]:
2    A
1    B
0    C
dtype: object
In [102]:
s[0]

Out[102]:
'C'

We can always convert a Series back to a dict. The pandas indexes become dict keys.

In [103]:
# Series to dict
dict(s)

Out[103]:
{2: 'A', 1: 'B', 0: 'C'}

## Series vs. List¶

We can also convert back and forth between lists as Series:

In [108]:
num_list = [100, 200, 300]
print(type(num_list))

num_series = Series(num_list) # create Series from list
print(type(num_series))

<class 'list'>
<class 'pandas.core.series.Series'>

In [109]:
# displaying a list:
num_list

Out[109]:
[100, 200, 300]
In [110]:
# displaying a Series:
num_series

Out[110]:
0    100
1    200
2    300
dtype: int64

Notice that both the list and the Series contain the same values. However, there are some differences:

• the Series is displayed vertically
• the indexes for the series are explicitly displayed by the values
• at the end, it says "dtype: int64"

When we create a Series from a list, there is no difference between the integer position and index. s.iloc[X] is the same as s.loc[X].

In [112]:
num_series.iloc[0], num_series.loc[0]

Out[112]:
(100, 100)

Going from a Series back to a list is just as easy as going from a list to a Series:

In [113]:
list(num_series)

Out[113]:
[100, 200, 300]

## Indexing and Slicing¶

Except for negative indexing, indexing and slicing for a Series is much like it is for a list.

In [14]:
letter_list = ["A", "B", "C", "D"]
letter_series = Series(letter_list)
letter_series

Out[14]:
0    A
1    B
2    C
3    D
dtype: object
In [15]:
letter_list[0]

Out[15]:
'A'
In [16]:
letter_series[0]

Out[16]:
'A'
In [17]:
letter_list[3]

Out[17]:
'D'
In [18]:
letter_series[3]

Out[18]:
'D'
In [19]:
letter_list[-1]

Out[19]:
'D'
In [114]:
# but be careful!  Series don't support negative indexes to the extent that lists do
try:
except Exception as e:
print(type(e))

letter_series.iloc[-1] # OK

<class 'KeyError'>

Out[114]:
'D'

Series slicing works much like list slicing:

In [115]:
print("list slice:")
print(letter_list[:2])
print("\nseries slice:")
print(letter_series[:2])

list slice:
['A', 'B']

series slice:
0    A
1    B
dtype: object

In [116]:
print("list slice:")
print(letter_list[2:])
print("\nseries slice:")
print(letter_series[2:])

list slice:
['C', 'D']

series slice:
2    C
3    D
dtype: object


Be careful! Notice the indices for the slice. It is not creating a new Series indexed from zero, as you would expect with a list.

In [117]:
# although we CANNOT always do negative indexing with a Series
# we CAN use negative numbers in a Series slice
print("list slice:")
print(letter_list[:-1])
print("\nseries slice:")
print(letter_series[:-1])

list slice:
['A', 'B', 'C']

series slice:
0    A
1    B
2    C
dtype: object


You should think of Series(["A", "B", "C"]) as being similar to this:

In [118]:
s = Series({0: "A", 1: "B", 2: "C"})
s

Out[118]:
0    A
1    B
2    C
dtype: object

We can also slice a Series constructed from a dictionary (remember that you may not slice a regular Python dict):

In [119]:
s[1:]

Out[119]:
1    B
2    C
dtype: object

## Element-Wise Operations¶

With Series, it is easy to apply the same operation to every value in the Series with a single line of code (instead of with a loop).

For example, suppose we wanted to add 1 to every item in a list. We would need to write something like this:

In [120]:
orig_nums = [100, 200, 300]
new_nums = []
for x in orig_nums:
new_nums.append(x+1)
new_nums

Out[120]:
[101, 201, 301]

With a Series, we can do the same like this:

In [121]:
nums = Series([100, 200, 300])
nums + 1

Out[121]:
0    101
1    201
2    301
dtype: int64

This probably feels more intuitive for those of you familar with vector math.

It also means multiplication means something very different for lists than for Series.

In [122]:
[1,2,3] * 3

Out[122]:
[1, 2, 3, 1, 2, 3, 1, 2, 3]
In [123]:
Series([1,2,3]) * 3

Out[123]:
0    3
1    6
2    9
dtype: int64

Whereas a "+" means concatenate for lists, it means element-wise addition for Series:

In [124]:
[10, 20] + [3, 4]

Out[124]:
[10, 20, 3, 4]
In [125]:
Series([10, 20]) + Series([3, 4])

Out[125]:
0    13
1    24
dtype: int64

One implication of this is that you might not get what you expect if you add Series of different sizes:

In [126]:
Series([10,20,30]) + Series([1,2])

Out[126]:
0    11.0
1    22.0
2     NaN
dtype: float64

The 10 gets added with the 1, and the 20 gets added with the 2, but there's nothing in the second series to add with 30. 30 plus nothing doesn't make sense, so Pandas gives "NaN". This stands for "Not a Number".

## Boolean Element-Wise Operation¶

Consider the following:

In [128]:
nums = Series([1,9,8,2])
nums

Out[128]:
0    1
1    9
2    8
3    2
dtype: int64
In [129]:
nums > 5

Out[129]:
0    False
1     True
2     True
3    False
dtype: bool

This example shows that you can do element-wise comparisons as well. The result is a Series of booleans. If the value in the original Series is greater than 5, we see True at the same position in the output Series. Otherwise, the value at the same position in the output Series is False.

We can also chain these operations together:

In [130]:
nums = Series([7,5,8,2,3])
nums

Out[130]:
0    7
1    5
2    8
3    2
4    3
dtype: int64
In [131]:
mod_2 = nums % 2
mod_2

Out[131]:
0    1
1    1
2    0
3    0
4    1
dtype: int64
In [132]:
odd = mod_2 == 1
odd

Out[132]:
0     True
1     True
2    False
3    False
4     True
dtype: bool

As you can see, we first obtained an integer Series (mod_2) by computing the value of every number modulo 2 (mod_2) will of course contain only 1's and 0's).

We then create a Boolean series (odd) by comparing the mod_2 series to 1.

If a number in the nums Series is odd, then the value at the same position in the odd series will be True.

## Data Alignment¶

Notice what happens when we create a series from a list:

In [38]:
Series([100,200,300])

Out[38]:
0    100
1    200
2    300
dtype: int64

We see the following:

• the first position has index 0 and value 100
• the second position has index 1 and value 200
• the third position has index 2 and value 300

One interesting difference between lists and Series is that with Series, the index does not always need to correspond so closely with the position; that's just a default that can be overridden. We've already seen one way to do this (creating a Series from a dict). We can also build a Series with two aligned lists (one for the values and one for the index).

In [136]:
nums1 = Series([100, 200, 300], index=[2,1,0])
nums1

Out[136]:
2    100
1    200
0    300
dtype: int64

Now we see indexes are assigned based on the argument we passed for index (not the position):

• the first position has index 2 and value 100
• the second position has index 1 and value 200
• the third position has index 0 and value 300

When we do element-wise operations between two Series, Pandas lines up the data based on index, not position. As a concrete example, consider three Series:

In [137]:
X = Series([100, 200, 300])
Y = Series([10, 20, 30])
Z = Series([10, 20, 30], index=[2,1,0])

In [138]:
X

Out[138]:
0    100
1    200
2    300
dtype: int64
In [139]:
Y

Out[139]:
0    10
1    20
2    30
dtype: int64
In [140]:
Z

Out[140]:
2    10
1    20
0    30
dtype: int64

Note: Y and Z are nearly the same (numbers 10, 20, and 30, in that order), except for the index. Let's see the difference between X+Y and Y+Z:

In [141]:
X+Y

Out[141]:
0    110
1    220
2    330
dtype: int64
In [142]:
X+Z

Out[142]:
0    130
1    220
2    310
dtype: int64

For X+Y, Pandas adds the number at index 0 in X (100) with the value at index 0 in Y (10), such that the value in the output at index 0 is 110.

For X+Z, Pandas adds the number at index 0 in X (100) with the value at index 0 in Y (30), such that the value in the output at index 0 is 130. It doesn't matter that the first number in Z is 10, because Pandas does element-wise operations based on index, not position.

## Boolean Indexing¶

We've seen this syntax before:

obj[X]

For a dictionary, X is a key, and for a list, X is an index. With a Series, X could be either of these things, or, interestingly, obj and X could both be a Series. In this last scenario, X must specifically be a Series of booleans. This type of lookup is often called "boolean indexing", or sometimes "fancy indexing."

In [143]:
letters = Series(["A", "B", "C", "D"])
letters

Out[143]:
0    A
1    B
2    C
3    D
dtype: object
In [144]:
bool_series = Series([True, True, False, False])
bool_series

Out[144]:
0     True
1     True
2    False
3    False
dtype: bool
In [145]:
# we can used the bool_series almost like an index
# to pull values out of letters:

letters[bool_series]

Out[145]:
0    A
1    B
dtype: object
In [146]:
# We could also create the Boolean Series on the fly:
letters[Series([True, True, False, False])]

Out[146]:
0    A
1    B
dtype: object
In [147]:
# Let's grab the last two letterrs:
letters[Series([False, False, True, True])]

Out[147]:
2    C
3    D
dtype: object
In [148]:
# Let's grab the first and last (can't do this with a slice):
letters[Series([True, False, False, True])]

Out[148]:
0    A
3    D
dtype: object

As with element wise operations, fancy indexing aligns both Series:

In [149]:
s = Series({"w": 6, "x": 7, "y": 8, "z": 9})
b = Series({"w": True, "x": False, "y": False, "z": True})
s[b]

Out[149]:
w    6
z    9
dtype: int64

## Combining Element-Wise Operations with Selection¶

As we just saw, we can use a Boolean series (let's call it B) to select values from another Series (let's call it S).

A common pattern is to create B by performing operation on S, then using B to select from S. Let's try doing this to pull all the numbers greater than 5 from a Series.

### Example 1: extract number >5¶

In [150]:
# we want to pull out 9 and 8
S = Series([1,9,2,3,8])
S

Out[150]:
0    1
1    9
2    2
3    3
4    8
dtype: int64
In [151]:
B = S > 5
B

Out[151]:
0    False
1     True
2    False
3    False
4     True
dtype: bool
In [152]:
# this will pull out values from S at index 1 and 4,
# because the values in B at index 1 and 4 are True
S[B]

Out[152]:
1    9
4    8
dtype: int64

### Example 2: extract upper case letters¶

Let's try to pull out all the upper case strings from a series:

In [153]:
words = Series(["APPLE", "boy", "CAT", "dog"])
words

Out[153]:
0    APPLE
1      boy
2      CAT
3      dog
dtype: object
In [154]:
# we can use .str.upper() to get upper case version of words
upper_words = words.str.upper()
upper_words

Out[154]:
0    APPLE
1      BOY
2      CAT
3      DOG
dtype: object
In [155]:
# B will be True where the original word equals the upper-case version
B = words == upper_words
B

Out[155]:
0     True
1    False
2     True
3    False
dtype: bool
In [156]:
# pull out the just words that were orginally uppercase
words[B]

Out[156]:
0    APPLE
2      CAT
dtype: object

We have done this example in several steps to illustrate what is happening, but it could have been simplified. Recall that B is words == upper_words. Thus we could have done this without ever storing a Boolean series in B:

In [157]:
words[words == upper_words]

Out[157]:
0    APPLE
2      CAT
dtype: object

Let's simplify one step further (instead of using upper_words, let's paste the expression we used to compute it earlier):

In [158]:
words[words == words.str.upper()]

Out[158]:
0    APPLE
2      CAT
dtype: object

### Example 3: extract odd numbers¶

Let's try to pull out all the odd numbers from this Series:

In [159]:
nums = Series([11,12,19,18,15,17])
nums

Out[159]:
0    11
1    12
2    19
3    18
4    15
5    17
dtype: int64

nums % 2 well produce a Series of 1's (for odd numbers) and 0's (for even numbers). Thus nums % 2 == 1 produces a Boolean Series of True's (for odd numbers) and False's (for even numbers). Let's use that Boolean Series to pull out the odd numbers:

In [63]:
nums[nums % 2 == 1]

Out[63]:
0    11
2    19
4    15
5    17
dtype: int64

### Example 4: using and and/or or¶

One might be able to perform operations like this in Pandas:

Series([True, False]) or Series([False, False])


Unfortunately, that doesn't work, because Python doesn't let modules like Pandas override the behavior of and and or. Instead, you must use & and | for these respectively.

Let's try to get the numbers between 10 and 20:

In [64]:
s = Series([5, 55, 11, 12, 999])
s

Out[64]:
0      5
1     55
2     11
3     12
4    999
dtype: int64
In [65]:
s >= 10

Out[65]:
0    False
1     True
2     True
3     True
4     True
dtype: bool
In [66]:
s <= 20

Out[66]:
0     True
1    False
2     True
3     True
4    False
dtype: bool
In [67]:
(s >= 10) & (s <= 20)

Out[67]:
0    False
1    False
2     True
3     True
4    False
dtype: bool
In [68]:
s[(s >= 10) & (s <= 20)]

Out[68]:
2    11
3    12
dtype: int64

Cool, we got all the numbers between 10 and 20! Notice we needed extra parentheses, though. & and | are high precedence, so we need those to make the logical operators occur last.

# Pandas DataFrame¶

Pandas will often be used to deal with tabular data (much as in Excel).

In many tables, all the data in the same column is similar, so Pandas represents each column in a table as a Series object. A table is represented as a DataFrame, which is just a collection of named Series (one for each column).

We can use a dictionary of aligned Series objects to create a dictionary. For example:

In [69]:
name_column = Series(["Alice", "Bob", "Cindy", "Dan"])
score_column = Series([100, 150, 160, 120])

table = DataFrame({'name': name_column, 'score': score_column})
table

Out[69]:
name score
0 Alice 100
1 Bob 150
2 Cindy 160
3 Dan 120

Or, if we want, we can create a DataFrame table from a dictionary of lists, and Pandas will implicitly create the Series for each column for us:

In [70]:
data = {"name": ["Alice", "Bob", "Cindy", "Dan"],
"score": [100, 150, 160, 120]}
df = DataFrame(data)
df

Out[70]:
name score
0 Alice 100
1 Bob 150
2 Cindy 160
3 Dan 120

## Accessing DataFrame Values¶

There are a few things we might want to do:

1. extract a column of data
2. extract a row of data
3. extract a single cell
4. modify a single cell
In [71]:
# we'll use the DataFrame of scores defined
# in the previous section
df

Out[71]:
name score
0 Alice 100
1 Bob 150
2 Cindy 160
3 Dan 120
In [72]:
# let's grab the name cell using DataFrame["COL NAME"]
df["name"]

Out[72]:
0    Alice
1      Bob
2    Cindy
3      Dan
Name: name, dtype: object
In [73]:
# or we could extract the score column:
df["score"]

Out[73]:
0    100
1    150
2    160
3    120
Name: score, dtype: int64
In [74]:
# if we want to generate some simple stats over a column,
# we can use .describe()
df["score"].describe()

Out[74]:
count      4.000000
mean     132.500000
std       27.537853
min      100.000000
25%      115.000000
50%      135.000000
75%      152.500000
max      160.000000
Name: score, dtype: float64
In [75]:
# lookup is done for columns by default (df[x] looks up column named x)
# we can also lookup a row, but we need to use df.loc[y].  ("loc" stands for location)
# for example, let's get Bob's row:
df.loc[1]

Out[75]:
name     Bob
score    150
Name: 1, dtype: object
In [76]:
# if we want a particular cell, we can use df.loc[row,col].
# for example, this is Bob's score:
df.loc[1, "score"]

Out[76]:
150
In [77]:
# we can also use this to modify cells:
df.loc[1, "score"] += 5
df

Out[77]:
name score
0 Alice 100
1 Bob 155
2 Cindy 160
3 Dan 120

Most of the time, we'll let Pandas directly load a CSV file to a DataFrame (instead of creating a dictionary of lists ourselves). We can easily do this with pd.read_csv(path) (recall that we imported pandas as import pandas as pd):

In [160]:
# movies is a DataFrame

# how many are there?
print("Number of movies:", len(movies))

Number of movies: 998

In [161]:
# it's large, but we can preview the first few with DataFrame.head()

Out[161]:
Index Title Genre Director Cast Year Runtime Rating Revenue
0 0 Guardians of the Galaxy Action,Adventure,Sci-Fi James Gunn Chris Pratt, Vin Diesel, Bradley Cooper, Zoe S... 2014 121 8.1 333.13
1 1 Prometheus Adventure,Mystery,Sci-Fi Ridley Scott Noomi Rapace, Logan Marshall-Green, Michael ... 2012 124 7.0 126.46M
2 2 Split Horror,Thriller M. Night Shyamalan James McAvoy, Anya Taylor-Joy, Haley Lu Richar... 2016 117 7.3 138.12M
3 3 Sing Animation,Comedy,Family Christophe Lourdelet Matthew McConaughey,Reese Witherspoon, Seth Ma... 2016 108 7.2 270.32
4 4 Suicide Squad Action,Adventure,Fantasy David Ayer Will Smith, Jared Leto, Margot Robbie, Viola D... 2016 123 6.2 325.02
In [162]:
# we can pull out Runtime minutes if we like
runtime = movies["Runtime"]

# it's still long (same length as movies), but let's preview the first 10 runtime minutes

Out[162]:
0    121
1    124
2    117
3    108
4    123
5    103
6    128
7     89
8    141
9    116
Name: Runtime, dtype: int64
In [163]:
# what is the mean runtime, in hours?
runtime.mean() / 60

Out[163]:
1.8861723446893788
In [164]:
# what if we want stats about movies from 2016?
# use .head() on results to make it shorter

Out[164]:
0    False
1    False
2     True
3     True
4     True
Name: Year, dtype: bool

Observe:

• 0 is False because the movie at index 0 is from 2014 (look earlier)
• 1 is False because the movie at index 1 is from 2012
• 2-4 are True because the movies at indexes 2-4 are from 2016
• ...

Let's pull out the movies from 2016 using this Boolean Series:

In [165]:
movies_2016 = movies[movies["Year"] == 2016]
print("there are " + str(len(movies_2016)) + " movies in 2016")

there are 296 movies in 2016

Out[165]:
Index Title Genre Director Cast Year Runtime Rating Revenue
2 2 Split Horror,Thriller M. Night Shyamalan James McAvoy, Anya Taylor-Joy, Haley Lu Richar... 2016 117 7.3 138.12M
3 3 Sing Animation,Comedy,Family Christophe Lourdelet Matthew McConaughey,Reese Witherspoon, Seth Ma... 2016 108 7.2 270.32
4 4 Suicide Squad Action,Adventure,Fantasy David Ayer Will Smith, Jared Leto, Margot Robbie, Viola D... 2016 123 6.2 325.02
5 5 The Great Wall Action,Adventure,Fantasy Yimou Zhang Matt Damon, Tian Jing, Willem Dafoe, Andy Lau 2016 103 6.1 45.13
6 6 La La Land Comedy,Drama,Music Damien Chazelle Ryan Gosling, Emma Stone, Rosemarie DeWitt, J.... 2016 128 8.3 151.06M
7 7 Mindhorn Comedy Sean Foley Essie Davis, Andrea Riseborough, Julian Barrat... 2016 89 6.4 0
8 8 The Lost City of Z Action,Adventure,Biography James Gray Charlie Hunnam, Robert Pattinson, Sienna Mille... 2016 141 7.1 8.01
9 9 Passengers Adventure,Drama,Romance Morten Tyldum Jennifer Lawrence, Chris Pratt, Michael Sheen,... 2016 116 7.0 100.01M
10 10 Fantastic Beasts and Where to Find Them Adventure,Family,Fantasy David Yates Eddie Redmayne, Katherine Waterston, Alison Su... 2016 133 7.5 234.02
11 11 Hidden Figures Biography,Drama,History Theodore Melfi Taraji P. Henson, Octavia Spencer, Janelle Mon... 2016 127 7.8 169.27M
In [166]:
# let's get some general stats about movies from 2016
movies_2016.describe()

Out[166]:
Index Year Runtime Rating
count 296.000000 296.0 296.000000 296.000000
mean 374.986486 2016.0 107.337838 6.433446
std 299.342658 0.0 17.438533 1.023419
min 2.000000 2016.0 66.000000 2.700000
25% 105.750000 2016.0 94.000000 5.800000
50% 297.000000 2016.0 106.000000 6.500000
75% 615.250000 2016.0 118.000000 7.200000
max 997.000000 2016.0 163.000000 8.800000

We see (among other things) that the average Runtime is 107.34 minutes.

# Conclusion¶

Data comes in many different forms, but tabular data is especially common. The Pandas module helps us work with tabular data and integrates with ipython, making it fast and easy to compute simple statistics over columns within our dataset. In this lesson, we learned to do the following:

• perform element-wise operations on Series
• use Pandas data alignment to do computation involving two Series
• select specific values from a Series using another Boolean Series via boolean indexing
• organize tabular data as a collection of Series in a DataFrame
• populate a DataFrame from a CSV file
In [ ]: