An Introduction To Python - Working With Data

An Introduction To Software
Development Using Python
Spring Semester, 2015
Class #23:
Working With Data

Data Formatting
• In the real world, data comes in many different
shapes, sizes, and encodings.
• This means that you have to know how to
manipulate and transform it into a common format
that will permit efficient processing, sorting, and
storage.
• Python has the tools that will allow you to do all of
this…
Image Credit: publicdomainvectors.org

Your Programming Challenge
• The Florida Polytechnic track team has just been
formed.
• The coach really wants the team to win the state
competition in its first year.
• He’s been recording their training results from the
600m run.
• Now he wants to know the top three fastest times
for each team member.
Image Credit: animals.phillipmartin.info

Here’s What The Data
Looks Like
• James
2-34,3:21,2.34,2.45,3.01,2:01,2:01,3:10,2-22
• Julie
2.59,2.11,2:11,2:23,3-10,2-23,3:10,3.21,3-21
• Mike
2:22,3.01,3:01,3.02,3:02,3.02,3:22,2.49,2:38
• Sara
2:58,2.58,2:39,2-25,2-55,2:54,2.18,2:55,2:55
Image Credit: www.dreamstime.com

1st Step: We Need To Get The Data
• Let’s begin by reading the data from each of
the files into its own list.
• Write a short program to process each file,
creating a list for each athlete’s data, and
display the lists on screen.
• Hint: Try splitting the data on the commas,
and don’t forget to strip any unwanted
whitespace.
1 Image Credit: www.clipartillustration.com

New Python Ideas
• data.strip().split(',')
This is called method chaining.
The first method, strip() , is applied to the line in data, which
removes any unwanted whitespace from the string.
Then, the results of the stripping are processed by the second
method, split(',') , creating a list.
The resulting list is then saved in the variable. In this way, the
methods are chained together to produce the required result.
It helps if you read method chains from left to right.
Image Credit: www.clipartpanda.com

Time To Do Some Sorting
• In-Place Sorting
– Takes your data, arranges it in the order you specify, and then replaces your
original data with the sorted version.
– The original ordering is lost. With lists, the sort() method provides in-place
sorting
– Example - original list: [1,3,4,6,2,5]
list after sorting: [1,2,3,4,5,6]
• Copy Sorting
– Takes your data, arranges it in the order you specify, and then returns a sorted
copy of your original data.
– Your original data’s ordering is maintained and only the copy is sorted. In
Python, the sorted() method supports copied sorting.
– Example - original list: [1,3,4,6,2,5]
list after sorting: [1,3,4,6,2,5]
new list: [1,2,3,4,5,6]
2 Image Credit: www.picturesof.net

What’s Our Problem?
• “-”, “.”, and “:” all have different ASCII values.
• This means that they are screwing up our
sort.
• Sara’s data:
['2:58', '2.58', '2:39’, '2-25', '2-55', '2:54’, '2.18', '2:55', '2:55']
• Python sorts the strings, and when it comes
to strings, a dash comes before a period,
which itself comes before a colon.
• Nonuniformity in the coach’s data is causing
the sort to fail.

Fixing The Coach’s Mistakes
• Let’s create a function called sanitize() , which
takes as input a string from each of the
athlete’s lists.
• The function then processes the string to
replace any dashes or colons found with a
period and returns the sanitized string.
• Note: if the string already contains a
period, there’s no need to sanitize it.
3 Image Credit: www.dreamstime.com

Code Problem: Lots and Lots of
Duplication
• Your code creates four lists to hold the data as read
from the data files.
• Then your code creates another four lists to hold the
sanitized data.
• And, of course, you’re stepping through lists all over
the place…
• There has to be a better way to write code like this.
Image Credit: www.canstockphoto.com

Transforming Lists
• Transforming lists is such a common requirement
that Python provides a tool to make the
transformation as painless as possible.
• This tool goes by the rather unwieldy name of
list comprehension.
• List comprehensions are designed to reduce the
amount of code you need to write when
transforming one list into another.
Image Credit: www.fotosearch.com

Steps In Transforming A List
• Consider what you need to do when you transform one list
into another. Four things have to happen. You need to:
1. Create a new list to hold the transformed data.
2. Iterate each data item in the original list.
3. With each iteration, perform the transformation.
4. Append the transformed data to the new list.
clean_sarah = []
for runTime in sarah:
clean_sarah.append(sanitize(runTime))
❶
❷ ❸
❹
Image Credit: www.cakechooser.com

List Comprehension
• Here’s the same functionality as a list comprehension, which
involves creating a new list by specifying the transformation
that is to be applied to each of the data items within an
existing list.
clean_sarah = [sanitize(runTime) for runTime in sarah]
Create new list
… by applying
a transformation
… to each
data item
… within an
existing list
Note: that the transformation has been reduced to a single line
of code. Additionally, there’s no need to specify the use of the append()
method as this action is implied within the list comprehension
4 Image Credit: www.clipartpanda.com

Congratulations!
• You’ve written a program that reads the
Coach’s data from his data files, stores his raw
data in lists, sanitizes the data to a uniform
format, and then sorts and displays the
coach’s data on screen. And all in ~25 lines of
code.
• It’s probably safe to show
the coach your output now.
Image Credit: vector-magz.com

Ooops – Forgot Why We Were
Doing All Of This: Top 3 Times
• We forgot to worry about what we were
actually supposed to be doing: producing the
three fastest times for each athlete.
• Oh, of course, there’s no place for any
duplicated times in our output.
Image Credit: www.clipartpanda.com

Two Ways To Access The
Time Values That We Want
• Standard Notation
– Specify each list item individually
• sara[0]
• sara[1]
• sara[2]
• List Slice
– sara[0:3]
– Access list items up to, but not including, item 3.
Image Credit: www.canstockphoto.com

The Problem With Duplicates
• Do we have a duplicate problem?
• Processing a list to remove duplicates is one area where a list
comprehension can’t help you, because duplicate removal is not a
transformation; it’s more of a filter.
• And a duplicate removal filter needs to examine the list being created as it
is being created, which is not possible with a list comprehension.
• To meet this new requirement, you’ll need to revert to regular list iteration
code.
James
2-34,3:21,2.34,2.45,3.01,2:01,2:01,3:10,2-22
5 Image Credit: www.mycutegraphics.com

Remove Duplicates With Sets
• The overriding characteristics of sets in Python are that the data items in a
set are unordered and duplicates are not allowed.
• If you try to add a data item to a set that already contains the data item,
Python simply ignores it.
• It is also possible to create and populate a set in one step. You can provide
a list of data values between curly braces or specify an existing list as an
argument to the set()
• Any duplicates in the james list will be ignored:
distances = set(james)
distances = {10.6, 11, 8, 10.6, "two", 7}
Duplicates will be ignored
Image Credit: www.pinterest.com

What Do We Do Now?
• To extract the data you need, replace all of
that list iteration code in your current program
with four calls to:
sorted(set(...))[0:3]
6 Image Credit: www.fotosearch.com

What’s In Your Python Toolbox?
print() math strings I/O IF/Else elif While For
DictionaryLists And/Or/Not Functions Files ExceptionSets

What We Covered Today
1. Read in data
2. Sorted it
3. Fixed coach’s mistakes
4. Transformed the list
5. Used List Comprehension
6. Used sets to get rid of
duplicates
Image Credit: http://www.tswdj.com/blog/2011/05/17/the-grooms-checklist/

What We’ll Be Covering Next Time
1. External Libraries
2. Data wrangling
Image Credit: http://merchantblog.thefind.com/2011/01/merchant-newsletter/resolve-to-take-advantage-of-these-5-e-commerce-trends/attachment/crystal-ball-fullsize/

An Introduction To Python - Working With Data

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