Up To Schedule - Back To Let the Computer Do the Work - Forward To Make Incremental Changes I
This lecture covers a number of topics under the heading of "writing code for people":
- naming variables in meaningful ways
- choosing appropriate data types
- writing effective comments
- breaking your code/script into small pieces
In some cases, we'll also look at the best practice of "don't repeat yourself".
These concepts are important and apply to any language. For today's lecture, we'll use python as the language, but you use a different language, you should think about how you would use these practices in your language. In particular, we will use the IPython interpreter, a version of python with some added features.
At the same time, this exercise will demonstrate one way to think about developing and designing code/scripts in a modular way.
This is not designed to teach python, per se, but use it as a relatively accessible language with features that help shine a light on best practices. For more on python, consider the following reference material.
- Dive into Python
- Software Carpentry's Python Lectures
- IPython: A System for Interactive Scientific Computing
- How to Think Like a Computer Scientist
For this lecture, we will return to the simulated survey data that
we used in the lecture on using the shell. This data was in many different
files, each distributed over many different directories. One of the exercises
was to place all of those files in a single directory named cleaneddata and
give them all the same extension.
However, all those data in different files does not make it easy for us use for additional analysis. It would be easier if that data was all in a single file. We could do that by simply concatenating all the files to make one single file, but this will make it less obvious which pieces of data belong together.
Therefore, for this exercise, we will create a single file with all this data, but with one line per file, with all the data from that file. The different pieces of data will appear in the same order in each line, and will be separated by commas. This is a so-called comma-separated values (or CSV) file and is a common simple way to represent tables of data. In fact, this type of file can be loaded into most spreadsheet software.
We chose this example because it represents a common task: amalgamating data from a number of files into a single table or database. By choosing a CSV file, it may resonate even more strongly since it means you can use such a file in a spreadsheet. Imagine the effort of making a spreadsheet from this set of more than 350 files. Although spreadsheets are frequently NOT the best way to process data, they are probably something with which you are familiar.
Pro-tip: Using two windows will make it much easier to follow this exercise.
Let's start with the big picture view of what this script needs to do:
- loop through a list of filenames
- parse each filename into a single data record
- save that record
- write a header in the correct CSV syntax
- loop through each record
- write that record in the correct CSV syntax
Written in python that is:
for filename in filelist:
data_record = parseFile(filename)
all_data.append(data_record)
csv_separator = ','
writeCSVHeader(column_labels,csv_separator)
for data_record in all_data:
writeCSVRow(column_labels,data_record,csv_separator)
Follow along
- Open a new file in your editor called
text2csv.py - Copy & paste the above code into that editor.
Let's review the best practices so far:
These 8 lines (including 2 blank lines) represent all the main steps that our script must accomplish. We have only 6 variables and 4 functions to track in our heads.
These lines don't really need comments since we have chosen expressive variable and function names in a short script that performs clear operations.
We have chosen meaningful names for each variable: filename, filelist,
data_record, all_data, column_labels, csv_separator. It is often
recommended that variables be given names that are nouns, indicating that they
represent things rather than actions.
The length of variable names is also important: too short and they loose some meaning; too long and they become prone to errors when typing them.
Short Exercise
Which of these are bad choices and why? Which are reasonable alternatives?
fnall_data_from_all_the_files_in_the_listcol_lblsrecordsepcol_labels
We have chosen meaningful names for each function: parseFile, writeCSVHeader,
writeCSVRow. It is often recommended that functions be given names that are
verbs, indiciating that they represent action rather than things.
We have used a consistent style that helps the reader distinguish between variables, in which words are separated by underscores, and functions, in which CamelCase is used.
At this point we haven't made very few decisions on data types:
- the
filelistvariable should be some type that allows us to take advantage of python's ability to iterate over the members - the
data_recordvariable should be some type that stores all the information from a single file - the
all_datavariable should be some type that allows us to use theappend()method - the
column_labelsvariable should store all the column labels for our CSV file
Now is a good time to explore the different compound data types in python. Most languages have some compound data types and you should learn about them and their features to make the best choices in your scripts/code in that langauge.
For our problem the following choices are probably wise:
filelistshould be a list- lists are a common default unless you need other features like key-value pairs (dictionaries), immutability (tuples), or uniqueness (sets)
data_recordshould be a dictionary- each record will store the data that is already in key-value pairs in the file
all_datashould be a list of those dictionariescolumn_labelsshould be a tuple because we want it to be immutable
At this point there are two examples of using this best practice. In it's simplest form, we have used loops so that the computer repeats the same task for many different items.
In addition, using the csv_separator variable instead of just typing ','
has two advantages:
- it provides context for what this means
- it means we can change it in a single place
- We can initialize the list of files from the command-line arguments.
- We should also declare the immutable ordered list of column labels.
- Now that we know
all_datashould be a list, we should initialize it as a empty list.
Follow Along
Add these lines above the others in your script:
import sys
filelist = sys.argv[1:]
column_labels = ("ID","Reported","Age","Gender","State of residence","Income","Education","Hours per week")
all_data = []To continue with our script we can now focus on the first function,
parseFile. Python functions have the same components of functions in most
languages.
- a way to declare where the function starts:
def - a function name
- arguments to the function
- a way to return some result(s) from a function
- a way to declare where the function ends: indentation
For our function, we already know that the name is parseFile and that it
will take a single argument that is a filename.
Just like the last time, we'll consider what the big picture steps are:
- open the file
- loop over all the lines in the file
- extract the data from that line in a key/value pair
- add that data to the dictionary
- return the complete dictionary
Or written as python:
def parseFile(filename):
"""Read all the lines from a file and return them as a dictionary of key/value pairs"""
textfile = open(filename, 'r')
data_record = {}
for line in textfile:
# all lines that contain key/value pairs must have at least one colon
if ':' in line:
(key,value) = extractData(line)
data_record[key] = value
return data_record Follow along
- Copy & paste the above code into your file at the top.
At this point we've had to introduce a new concept: reading data from files.
For most purposes, python makes this very easy. When we open a file, we get
an object over which we can iterate, much like a list. Let's look at the file
phonenums.txt using this concept:
Try it exercise
In [15]: file_data = open('phonenums.txt')
In [16]: for line in file_data:
....: print lineAgain we'll review the best practices.
Once again we have a short method with only 10 lines, including 2 comments, and only 6 variables. To keep this method focused, we will rely on another new function.
In this case there are 2 places where comments are useful. Every function in
python should have a special comment at the beginning known as a docstring.
A docstring has special meaning to tools that, for example, can show help
about a function.
This function also requires a special test to ensure that the line is actually a data line, using the assumption that all data lines have at least one colon (:). This is a useful place for a comment because the conditional clause is not obvious without it.
Two of the variables used in this function use the same name and same meaning
as the corresponding variables in the main script: filename and
data_record. Although this is not necessary, it makes code more readable
when variables in different places that have the same meaning can also have
the same name. In functions that broader utility, this may be neither
possible nor logical.
The other variables have names that are obvious matches to their purposes:
textfile, line, key, and value. These are all nouns and the only
method that we have declared is a verb: extractData.
One of the features of python that we may start to notice is that it relies on something called dynamic typing. Although the type of a variable can change over its lifetime, too much change can make the script more difficult to follow. It is helpful to know what type is implied and endeavor to use that consistently.
In addition, the type of textfile is a built-in object of type file,
determined dynamically by the object returned by the call to the open
method.
Because data_record is a new dictionary each time we call this function, we
need to initialize it as a new dictionary each time.
The next function that we will add will operate on a small amount of data to separate a single line of the form:
Some keyword phrase: data that is the value
into two variables where:
key = "Some keyword phrase"
value = "data that is the value"
The line of data is a string, so we'll be using some methods of the built-in
string class. There are many ways to accomplish this task, but we'll rely on
the split() method of the string class with the following high level algorithm:
- split the line at the location of the separator into a list of strings
- use the first of this list as the key
- use the remainder of the list as the value
def extractData(line):
"""Assume the first colon (:) separates the key from the value."""
separator = ':'
line_data = line.strip().split(separator)
key = line_data[0]
# the value may contain a separator so will need to be reassembled from
# possibly multiple elements
value = separator.join(line_data[1:]).strip()
return key,value Follow along
- Copy & paste the above code into your file at the top.
One of the features of IPython is it's ability to show you help of a function,
typically in the form of its docstring. You can learn more about the
strip() and split() string functions:
In [21]: str.strip?
In [22]: str.split?
Let's again review the best practices:
Not much new to say here: a short method: 9 lines including 3 comments, and only 5 variables. At this point, we have used 12 different variables throughout the script, but no single block of the script uses more than 6 of them.
In addition to a docstring, we have a comment to remind us that when the
separator happens to appear as part of the value, the line_data list will
have more than just 2 elements, and we need to reassemble all but the first
one into the value.
Of the 5 variables used in this function, 3 are based on names defined and
discussed in previous functions. The separator variable is an appropriate
length and expressive in describing its role. The list line_data is a
descriptive name, but other names would be fine.
Think aloud: Recommend some alternative variable names for line_data.
All the choices in this case were made for us by the requirements of the string functions we call.
Discussion Exercise Given the following implementation of the final two
functions, discuss ways in which they follow the best practices, and ways
that they could be improved, if any.
def writeCSVHeader(column_labels,csv_separator):
header = []
for column in column_labels:
header.append('"' + column + '"')
print csv_separator.join(header)
def writeCSVRow(column_labels,data_record,csv_separator):
row = []
for column in column_labels:
row.append('"' + data_record[column] + '"')
print csv_separator.join(row)To run this script you can either call it from the command line or from within the IPython interpreter, with an argument list made up of many of the original data files.
From the command line:
python text2csv.py ../../shell/data/Bert/*
From the iPython interpreter:
In [12]: %run text2csv.py ../../shell/data/Bert/*
##Writing Code for People summary
We learned some basics of python and saw that variable type, name, comments and white space affect more than just code functionality, they affect the readability for others and your future self. Variable names can make a huge difference in code readability and types are important in conveying intent.
Up To Schedule - Back To Let the Computer Do the Work - Forward To Make Incremental Changes I