After reading a couple of posts by Michael Lopez about the NFL draft, I decided to recreate some of his analysis using Python (instead of R).

First up lets import most of the stuff we will be using.

NOTE: You can find the github repository for this blog post here. It contains this notebook, the data and the conda environment I used.

In [1]:
%matplotlib inline

import matplotlib.pyplot as plt
import seaborn as sns
import pandas as pd

from urllib.request import urlopen
from bs4 import BeautifulSoup


# Web Scraping¶

Before we can do anything we will need some data. We'll be scraping draft data from Pro-Football-Reference and then cleaning it up for the analysis.

We will use BeautifulSoup to scrape the data and then store it into a pandas Dataframe.

To get a feel of the data lets take a look at the 1967 draft.

Above is just a small part of the draft table found on the web page. We will extract the second row of column headers and all information for each pick. While we we are at it, we will also scrape each player's Pro-Football-Reference player page link and college stats link. This way if we ever want to scrape data from their player page in the future, we can.

In [2]:
# The url we will be scraping
url_1967 = "http://www.pro-football-reference.com/years/1967/draft.htm"

# get the html
html = urlopen(url_1967)

# create the BeautifulSoup object
soup = BeautifulSoup(html, "lxml")


The column headers we need for our DataFrame are found in the second row of column headers PFR table. We will will scrape those and add two additional columns headers for the two additional player page links.

In [3]:
# Extract the necessary values for the column headers from the table
# and store them as a list
column_headers = [th.getText() for th in
soup.findAll('tr', limit=2)[1].findAll('th')]



## Scraping the Data¶

We can easily extract the rows of data using the CSS selector "#draft tr". What we are essentially doing is selecting table row elements within the HTML element that has the id value "draft".

A really helpful tool when it comes to finding CSS selectors is SelectorGadget. It's a web extension that lets you click on different elements of a web page and provides the CSS selector for those selected elements.

In [4]:
# The data is found within the table rows of the element with id=draft
# We want the elements from the 3rd row and on
table_rows = soup.select("#drafts tr")[2:]


Note that table_rows is a list of tag elements.

In [5]:
type(table_rows)

Out[5]:
list
In [6]:
type(table_rows[0])

Out[6]:
bs4.element.Tag
In [7]:
table_rows[0] # take a look at the first row

Out[7]:

1
1
BAL
Bubba Smith
DE
22
1976
1
2
6
62
46
111

Michigan St.
College Stats


The data we want for each player is found within the the td (or table data) elements.

Below I've created a function that extracts the data we want from table_rows. The comments should walk you through what each part of the function does.

In [8]:
def extract_player_data(table_rows):
"""
Extract and return the the desired information from the td elements within
the table rows.
"""
# create the empty list to store the player data
player_data = []

for row in table_rows:  # for each row do the following

# Get the text for each table data (td) element in the row
# Some player names end with ' HOF', if they do, get the text excluding
# those last 4 characters,
# otherwise get all the text data from the table data
player_list = [td.get_text()[:-4] if td.get_text().endswith(" HOF")
else td.get_text() for td in row.find_all("td")]

# there are some empty table rows, which are the repeated
# column headers in the table
# we skip over those rows and and continue the for loop
if not player_list:
continue

# Instead of a list we create a dictionary, this way we can easily
# match the player name with their pfr url
# For all "a" elements in the row, get the text
# NOTE: Same " HOF" text issue as the player_list above
if link.get_text().endswith(" HOF")  # if they are " HOF"
# else get all text, set thet as the dictionary key
# and set the url as the value

# The data we want from the dictionary can be extracted using the
# player's name, which returns us their pfr url, and "College Stats"
# which returns us their college stats page

# or en empty string if there is no link

# add the link for the player's college stats or an empty string
# if ther is no link

# Now append the data to list of data
player_data.append(player_list)

return player_data


Now we can create a DataFrame with the data from the 1967 draft.

In [9]:
# extract the data we want
data = extract_player_data(table_rows)

# and then store it in a DataFrame

In [10]:
df_1967.head()

Out[10]:
Rnd Pick Tm Pos Age To AP1 PB St ... TD Rec Yds TD Int Sk College/Univ Player_NFL_Link Player_NCAA_Link
0 1 1 BAL Bubba Smith DE 22 1976 1 2 6 ... Michigan St. College Stats /players/S/SmitBu00.htm http://www.sports-reference.com/cfb/players/bu...
1 1 2 MIN Clint Jones RB 22 1973 0 0 2 ... 20 38 431 0 Michigan St. College Stats /players/J/JoneCl00.htm http://www.sports-reference.com/cfb/players/cl...
2 1 3 SFO Steve Spurrier QB 22 1976 0 0 6 ... 2 Florida College Stats /players/S/SpurSt00.htm http://www.sports-reference.com/cfb/players/st...
3 1 4 MIA Bob Griese QB 22 1980 2 8 12 ... 7 Purdue College Stats /players/G/GrieBo00.htm http://www.sports-reference.com/cfb/players/bo...
4 1 5 HOU George Webster LB 21 1976 3 3 6 ... 5 Michigan St. College Stats /players/W/WebsGe00.htm http://www.sports-reference.com/cfb/players/ge...

5 rows × 30 columns

### Scraping the Data for All Seasons Since 1967¶

Scraping the for all drafts since 1967 follows is essentially the same process as above, just repeated for each draft year, using a for loop.

As we loop over the years, we will create a DataFrame for each draft, and append it to a large list of DataFrames that contains all the drafts. We will also have a separate list that will contain any errors and the url associated with that error. This will let us know if there are any issues with our scraper, and which url is causing the error. We will also have to add an additional column for tackles. Tackles show up after the 1993 season, so that is a column we need to insert into the DataFrames we create for the drafts from 1967 to 1993.

In [11]:
# Create an empty list that will contain all the dataframes
# (one dataframe for each draft)
draft_dfs_list = []

# a list to store any errors that may come up while scraping
errors_list = []

In [12]:
# The url template that we pass in the draft year inro
url_template = "http://www.pro-football-reference.com/years/{year}/draft.htm"

# for each year from 1967 to (and including) 2016
for year in range(1967, 2017):

# Use try/except block to catch and inspect any urls that cause an error
try:
# get the draft url
url = url_template.format(year=year)

# get the html
html = urlopen(url)

# create the BeautifulSoup object
soup = BeautifulSoup(html, "lxml")

column_headers = [th.getText() for th in
soup.findAll('tr', limit=2)[1].findAll('th')]

# select the data from the table using the '#drafts tr' CSS selector
table_rows = soup.select("#drafts tr")[2:]

# extract the player data from the table rows
player_data = extract_player_data(table_rows)

# create the dataframe for the current years draft

# if it is a draft from before 1994 then add a Tkl column at the
# 24th position
if year < 1994:
year_df.insert(24, "Tkl", "")

# add the year of the draft to the dataframe
year_df.insert(0, "Draft_Yr", year)

# append the current dataframe to the list of dataframes
draft_dfs_list.append(year_df)

except Exception as e:
# Store the url and the error it causes in a list
error =[url, e]
# then append it to the list of errors
errors_list.append(error)

In [13]:
len(errors_list)

Out[13]:
0
In [14]:
errors_list

Out[14]:
[]

We don't get any errors, so that's good.

Now we can concatenate all the DataFrames we scraped and create one large DataFrame containing all the drafts.

In [15]:
# store all drafts in one DataFrame
draft_df = pd.concat(draft_dfs_list, ignore_index=True)

In [16]:
# Take a look at the first few rows

Out[16]:
Draft_Yr Rnd Pick Tm Pos Age To AP1 PB ... Rec Yds TD Tkl Int Sk College/Univ Player_NFL_Link Player_NCAA_Link
0 1967 1 1 BAL Bubba Smith DE 22 1976 1 2 ... Michigan St. College Stats /players/S/SmitBu00.htm http://www.sports-reference.com/cfb/players/bu...
1 1967 1 2 MIN Clint Jones RB 22 1973 0 0 ... 38 431 0 Michigan St. College Stats /players/J/JoneCl00.htm http://www.sports-reference.com/cfb/players/cl...
2 1967 1 3 SFO Steve Spurrier QB 22 1976 0 0 ... Florida College Stats /players/S/SpurSt00.htm http://www.sports-reference.com/cfb/players/st...
3 1967 1 4 MIA Bob Griese QB 22 1980 2 8 ... Purdue College Stats /players/G/GrieBo00.htm http://www.sports-reference.com/cfb/players/bo...
4 1967 1 5 HOU George Webster LB 21 1976 3 3 ... 5 Michigan St. College Stats /players/W/WebsGe00.htm http://www.sports-reference.com/cfb/players/ge...

5 rows × 32 columns

We should edit the columns a bit as there are some repeated column headers and some are even empty strings.

In [17]:
# get the current column headers from the dataframe as a list

# The 5th column header is an empty string, but represesents player names

# Prepend "Rush_" for the columns that represent rushing stats

# Prepend "Rec_" for the columns that reperesent receiving stats

# Properly label the defensive int column as "Def_Int"

# Just use "College" as the column header represent player's colleger or univ

# Take a look at the updated column headers

Out[17]:
['Draft_Yr',
'Rnd',
'Pick',
'Tm',
'Player',
'Pos',
'Age',
'To',
'AP1',
'PB',
'St',
'CarAV',
'DrAV',
'G',
'Cmp',
'Att',
'Yds',
'TD',
'Int',
'Rush_Att',
'Rush_Yds',
'Rush_TD',
'Rec',
'Rec_Yds',
'Rec_TD',
'Tkl',
'Def_Int',
'Sk',
'College',
'',
'Player_NCAA_Link']
In [18]:
# Now assign edited columns to the DataFrame


Now that we fixed up the necessary columns, let's write out the raw data to a CSV file.

In [19]:
# Write out the raw draft data to the raw_data fold in the data folder
draft_df.to_csv("data/raw_data/pfr_nfl_draft_data_RAW.csv", index=False)


# Cleaning the Data¶

Now that we have the raw draft data, we need to clean it up a bit in order to do some of the data exploration we want.

First lets create a separate DataFrame that contains the player names, their player page links, and the player ID on Pro-Football-Reference. This way we can have a separate CSV file that just contains the necessary information to extract individual player data for Pro-Football-Reference sometime in the future.

To extract the Pro-Football-Reference player ID from the player link, we will need to use a regular expression. Regular expressions are a sequence of characters used to match some pattern in a body of text. The regular expression that we can use to match the pattern of the player link and extract the ID is as follows:

/.*/.*/(.*)\.



What the above regular expression essentially says is match the string with the following pattern:

• One '/'.
• Followed by 0 or more characters (this is represented by the '.*' characters).
• Followed by another '/' (the 2nd '/' character).
• Followed by 0 or more characters (again the '.*' characters) .
• Followed by another (3rd) '/'.
• Followed by a grouping of 0 or more characters (the '(.*)' characters).
• This is the key part of our regular expression. The '()' characters create a grouping around the characters we want to extract. Since the player IDs are found between the 3rd '/' and the '.', we use '(.*)' to extract all the characters found in that part of our string.
• Followed by a '.', character after the player ID.

We can extract the IDs by passing the above regular expression into the pandas extract method.

In [20]:
# extract the player id from the player links
# expand=False returns the IDs as a pandas Series
expand=False)

In [21]:
# add a Player_ID column to our draft_df
draft_df["Player_ID"] = player_ids

In [22]:
# add the beginning of the pfr url to the player link column
pfr_url = "http://www.pro-football-reference.com"


Now we can save a DataFrame just containing the player names, IDs, and links.

In [23]:
# Get the Player name, IDs, and links
player_id_df = draft_df.loc[:, ["Player", "Player_ID", "Player_NFL_Link",
# Save them to a CSV file
index=False)


## Cleaning Up the Rest of the Draft Data¶

Now that we are done with the play ID stuff lets get back to dealing with the draft data.

Lets first drop some unnecessary columns.

In [24]:
# drop the the player links and the column labeled by an empty string
draft_df.drop(draft_df.columns[-4:-1], axis=1, inplace=True)


The main issue left with the rest of the draft data is converting everything to their proper data type.

In [25]:
draft_df.info()


Int64Index: 15845 entries, 0 to 15844
Data columns (total 30 columns):
Draft_Yr     15845 non-null int64
Rnd          15845 non-null object
Pick         15845 non-null object
Tm           15845 non-null object
Player       15845 non-null object
Pos          15845 non-null object
Age          15845 non-null object
To           15845 non-null object
AP1          15845 non-null object
PB           15845 non-null object
St           15845 non-null object
CarAV        15845 non-null object
DrAV         15845 non-null object
G            15845 non-null object
Cmp          15845 non-null object
Att          15845 non-null object
Yds          15845 non-null object
TD           15845 non-null object
Int          15845 non-null object
Rush_Att     15845 non-null object
Rush_Yds     15845 non-null object
Rush_TD      15845 non-null object
Rec          15845 non-null object
Rec_Yds      15845 non-null object
Rec_TD       15845 non-null object
Tkl          15845 non-null object
Def_Int      15845 non-null object
Sk           15845 non-null object
College      15845 non-null object
Player_ID    11416 non-null object
dtypes: int64(1), object(29)
memory usage: 3.7+ MB


From the above we can see that a lot of the player data isn't numeric when it should be. To convert all the columns to their proper numeric type we can apply the to_numeric function to the whole DataFrame. Because it is impossible to convert some of the columns (e.g. Player, Tm, etc.) into a numeric type (since they aren't numbers) we need to set the errors parameter to "ignore" to avoid raising any errors.

In [26]:
# convert the data to proper numeric types
draft_df = draft_df.apply(pd.to_numeric, errors="ignore")

In [27]:
draft_df.info()


Int64Index: 15845 entries, 0 to 15844
Data columns (total 30 columns):
Draft_Yr     15845 non-null int64
Rnd          15845 non-null int64
Pick         15845 non-null int64
Tm           15845 non-null object
Player       15845 non-null object
Pos          15845 non-null object
Age          11297 non-null float64
To           10995 non-null float64
AP1          15845 non-null int64
PB           15845 non-null int64
St           15845 non-null int64
CarAV        10995 non-null float64
DrAV         9571 non-null float64
G            10962 non-null float64
Cmp          1033 non-null float64
Att          1033 non-null float64
Yds          1033 non-null float64
TD           1033 non-null float64
Int          1033 non-null float64
Rush_Att     2776 non-null float64
Rush_Yds     2776 non-null float64
Rush_TD      2776 non-null float64
Rec          3395 non-null float64
Rec_Yds      3395 non-null float64
Rec_TD       3395 non-null float64
Tkl          3644 non-null float64
Def_Int      2590 non-null float64
Sk           2670 non-null float64
College      15845 non-null object
Player_ID    11416 non-null object
dtypes: float64(19), int64(6), object(5)
memory usage: 3.7+ MB


We are not done yet. A lot of out numeric columns are missing data because players didn't accumulate any of those stats. For example, some players didn't score a TD or even play a game. Let's select the columns with numeric data and then replace the NaNs (the current value that represents the missing data) with 0s, as that is a more appropriate value.

In [28]:
# Get the column names for the numeric columns
num_cols = draft_df.columns[draft_df.dtypes != object]

# Replace all NaNs with 0
draft_df.loc[:, num_cols] = draft_df.loc[:, num_cols].fillna(0)

In [29]:
# Everything is filled, except for Player_ID, which is fine for now
draft_df.info()


Int64Index: 15845 entries, 0 to 15844
Data columns (total 30 columns):
Draft_Yr     15845 non-null int64
Rnd          15845 non-null int64
Pick         15845 non-null int64
Tm           15845 non-null object
Player       15845 non-null object
Pos          15845 non-null object
Age          15845 non-null float64
To           15845 non-null float64
AP1          15845 non-null int64
PB           15845 non-null int64
St           15845 non-null int64
CarAV        15845 non-null float64
DrAV         15845 non-null float64
G            15845 non-null float64
Cmp          15845 non-null float64
Att          15845 non-null float64
Yds          15845 non-null float64
TD           15845 non-null float64
Int          15845 non-null float64
Rush_Att     15845 non-null float64
Rush_Yds     15845 non-null float64
Rush_TD      15845 non-null float64
Rec          15845 non-null float64
Rec_Yds      15845 non-null float64
Rec_TD       15845 non-null float64
Tkl          15845 non-null float64
Def_Int      15845 non-null float64
Sk           15845 non-null float64
College      15845 non-null object
Player_ID    11416 non-null object
dtypes: float64(19), int64(6), object(5)
memory usage: 3.7+ MB


We are finally done cleaning the data and now we can save it to a CSV file.

In [30]:
draft_df.to_csv("data/clean_data/pfr_nfl_draft_data_CLEAN.csv", index=False)


# Exploring the NFL Draft¶

Now that we are done getting and cleaning the data we want, we can finally have some fun. First lets just keep the draft data up to and including the 2010 draft, as players who have been drafted more recently haven't played enough to accumulate have a properly representative career Approximate Value (or cAV).

In [31]:
# get data for drafts from 1967 to 2010
draft_df_2010 = draft_df.loc[draft_df.Draft_Yr <= 2010, :]

In [32]:
draft_df_2010.tail() # we see that the last draft is 2010

Out[32]:
Draft_Yr Rnd Pick Tm Player Pos Age To AP1 PB ... Rush_Yds Rush_TD Rec Rec_Yds Rec_TD Tkl Def_Int Sk College Player_ID
14314 2010 7 251 OAK Stevie Brown DB 23.0 2014.0 0 0 ... 0.0 0.0 0.0 0.0 0.0 98.0 8.0 1.0 Michigan BrowSt99
14315 2010 7 252 MIA Austin Spitler LB 23.0 2013.0 0 0 ... 0.0 0.0 0.0 0.0 0.0 11.0 0.0 0.0 Ohio St. SpitAu99
14316 2010 7 253 TAM Erik Lorig DE 23.0 2014.0 0 0 ... 4.0 0.0 39.0 220.0 2.0 3.0 0.0 0.0 Stanford LoriEr99
14317 2010 7 254 STL Josh Hull LB 23.0 2013.0 0 0 ... 0.0 0.0 0.0 0.0 0.0 11.0 0.0 0.0 Penn St. HullJo99
14318 2010 7 255 DET Tim Toone WR 25.0 2012.0 0 0 ... 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Weber St. ToonTi00

5 rows × 30 columns

## The Career Approximate Value distribution¶

Using seaborn's distplot function we can quickly view the shape of the cAV distribution as both a histogram and kernel density estimation plot.

In [33]:
# set some plotting styles
from matplotlib import rcParams

# set the font scaling and the plot sizes
sns.set(font_scale=1.65)
rcParams["figure.figsize"] = 12,9

In [34]:
# Use distplot to view the distribu
sns.distplot(draft_df_2010.CarAV)
plt.title("Distribution of Career Approximate Value")
plt.xlim(-5,150)
plt.show()


We can also view the distributions by position via the boxplot function.

In [35]:
sns.boxplot(x="Pos", y="CarAV", data=draft_df_2010)
plt.title("Distribution of Career Approximate Value by Position (1967-2010)")
plt.show()


From both of the above plots we see that most players don't end up doing much in their NFL careers as most players hover around the 0-10 cAV range.

There are also some positions that have a 0 cAV for the whole distribution or a very low (and small) cAV distribution. As we can see from the value counts below, that's probably due to the fact that there are very few players with those position labels.

In [36]:
# Look at the counts for each position
draft_df_2010.Pos.value_counts()

Out[36]:
DB    2456
LB    1910
RB    1686
WR    1636
DE    1130
T     1091
G      959
DT     889
TE     802
QB     667
C      425
K      187
P      150
NT     127
FB      84
FL      63
E       29
HB      23
KR       3
WB       2
Name: Pos, dtype: int64

Let's drop those position and merge the "HB" players with the "RB" players.

In [37]:
# drop players from the following positions [FL, E, WB, KR]
drop_idx = ~ draft_df_2010.Pos.isin(["FL", "E", "WB", "KR"])

draft_df_2010 = draft_df_2010.loc[drop_idx, :]

In [38]:
# Now replace HB label with RB label
draft_df_2010.loc[draft_df_2010.Pos == "HB", "Pos"] = "RB"


Lets take a look at the positional distributions again.

In [39]:
sns.boxplot(x="Pos", y="CarAV", data=draft_df_2010)
plt.title("Distribution of Career Approximate Value by Position (1967-2010)")
plt.show()


## Fitting a Draft Curve¶

Now we can fit a curve to take a look at the cAV at each pick. We will fit the curve using local regression, which "travels" along the data fitting a curve to small chunks of the data at a time. A cool visualization (from the Simply Statistics blog) of this process can be seen below:

seaborn lets us plot a Lowess curve pretty easily by using regplot and setting the lowess parameter to True.

In [40]:
# plot LOWESS curve
# set line color to be black, and scatter color to cyan
sns.regplot(x="Pick", y="CarAV", data=draft_df_2010, lowess=True,
line_kws={"color": "black"},
scatter_kws={"color": sns.color_palette()[5], "alpha": 0.5})
plt.title("Career Approximate Value by Pick")
plt.xlim(-5, 500)
plt.ylim(-5, 200)
plt.show()


We can also fit a Lowess curve for each position using lmplot and setting hue to "Pos".

In [41]:
# Fit a LOWESS curver for each position
sns.lmplot(x="Pick", y="CarAV", data=draft_df_2010, lowess=True, hue="Pos",
size=10, scatter=False)
plt.title("Career Approximate Value by Pick and Position")
plt.xlim(-5, 500)
plt.ylim(-1, 60)
plt.show()


The above plot is a bit too messy as there are too many lines. We can actually separate the curves out and plot the position curves individually. To do this instead of setting hue to "Pos" we can set col to "Pos". To organize the plots in 5x3 grid all we must set col_wrap to 5.

In [44]:
lm = sns.lmplot(x="Pick", y="CarAV", data=draft_df_2010, lowess=True, col="Pos",
col_wrap=5, size=4, line_kws={"color": "black"},
scatter_kws={"color": sns.color_palette()[5], "alpha": 0.7})

# add title to the plot (which is a FacetGrid)
lm.fig.suptitle("Career Approximate Value by Pick and Position",
fontsize=30)

plt.xlim(-5, 500)
plt.ylim(-1, 100)
plt.show()