Data Flow
Computers read code line by line, top to bottom of a script. But what if you want to have code run not in sequential order, or you want your code to do something depending on a value, or you want to reuse your code and run it multiple times?
We can solve all those problems with data flow structures.
In code_review_week2.py you had to print out multiple values from the tools dictionary. While typing each value out is tedious, it was still possible to do. However, what would happen if you had hundreds or thousands of values?
There's a much faster way to traverse data structures and types in Python, called Looping. With various types of loops in Python, you can travel through a sequence (i.e. a list, dictionary, string, etc...) to be able to manipulate items within the sequence.
For Loops are one of the most common ways in python to loop over a sequence. But what does looping mean exactly?
Let's go back to our script from week 2, and find our list of names. Add these lines to the script:
names=["Sara", "Kevin", "Shiva", "Anna", "Meher", "Maia"]
for name in names:
print(name)
We can also use For Loops on dictionaries. The syntax is slightly different because dictionaries are not one long sequence, but rather a sequence of key/value pairs.
tools = {"Python":{"2015":9, "2016":22, "2017":27, "2018":32, "2019":35}, "Javascript":{"2015":8, "2016":18, "2017":12, "2018":6, "2019":15}, "Twitter":{"2015":10, "2016":18, "2017":26, "2018":16, "2019":12}, "Github":{"2015":2, "2016":6, "2017":17, "2018":5, "2019":10}, "Gephi":{"2015":11, "2016":16, "2017":14, "2018":10, "2019":9}, "Geonames":{"2015":2, "2016":4, "2017":3, "2018":1, "2019":8}, "Transkribus":{"2015":0, "2016":1, "2017":2, "2018":1, "2019":8}, "Excel":{"2015":5, "2016":9, "2017":3, "2018":6, "2019":7}, "MySQL":{"2015":0, "2016":6, "2017":9, "2018":5, "2019":7}}
for key, value in tools.items():
print('key', key)
print('value', value)Or even strings.
for letter in 'Digital Humanities':
print(letter)For more about looping in Python checkout https://wiki.python.org/moin/ForLoop and https://www.learnpython.org/en/Loops.
Looping is very powerful, but if we want to loop through a second list of names or a different variable, we would have to repeat our code later in the script. An alternative is to create a function.
A function is a way to organize code by packaging many lines of code together into a single bundle. At the most basic level, this makes it easy to reuse code: it's easier to write out and read a single line rather than many lines and easier to change code in one place rather than in many places.
Function Syntax
To create a function, we define using def and a unique name and finally parentheses, followed by colon. Then we can pass arguments (also called parameters) in the parentheses, that we can that use inside of the functions. Those arguments will be variables and so we can do anything you would normally do to a value. Finally we can return the result of our manipulation.
For example try:
def get_fundamentals():
fundamentals = 'Having fun'
print(fundamentals)
return fundamentals
get_fundamentals()What would happen if you try to access the variable fundamentals after the line get_fundamentals()?
Spoiler alert you'll get this message:
NameError: name 'fundamentals' is not definedWe get that error because fundamentals is only defined in the local scope of the function. That is the variable fundamentals only exists within the function and can only be accessed within it. This is why when writing functions we either use two or four spaces (or tabs) to indent our code.
This is called "white space", and indicates to Python that everything that's indented is part of the function definition. As soon as we stop indenting, the function definition is over. We can use any number of spaces or tabs, but they have to be consistent. Notice we also used white space in the For Loops. Similar to in our function, variables defined within the for loop are only available within the local scope of the loop.
We also use something called a return statement.
def get_fundamentals():
fundamentals = 'Having fun'
print(fundamentals)
return fundamentals
new_fundamentals = get_fundamentals()If you printed out new_fundamentals what would this variable contain? You might expect it to print out the function, but it will actually print out the local variable fundamentals.
Return is used to pass data back from the function, which can then be assigned to a new variable that's accessible in the rest of the script, also known as the global scope.
def get_fundamentals(code_concept):
fundamentals = 'Having fun, ' + code_concept
return fundamentals
new_fundamentals = get_fundamentals('for loops')We can also pass in data to our functions through the parentheses when we call and define our functions. This is called passing arguments into our functions. In this example, what does new_fundamentals contain?
Arguments can have any name (so our example above could have named code_concept as data or x) and can contain any data type or structure.
Using functions and for loops we can reuse our code whenever we want within our script.
For example, try and understand this code from week 2 exercise:
def make_tool_dict(name, value_2015 , value_2016, value_2017,value_2018, value_2019):
return {
"2015":value_2015 ,
"2016":value_2016,
"2017":value_2017,
"2018":value_2018,
"2019":value_2019,
"name":name,
"total":value_2015+value_2016+value_2017+value_2018+value_2019
}
dh_tools=[make_tool_dict("Python",9,22,27,32,35),
make_tool_dict("JavaScript",8,18,12,6,15),
make_tool_dict("Twitter",10,18,26,16,12),
make_tool_dict("GitHub",2,6,17,5,10),
make_tool_dict("Gephi",11,16,14,10,9),
make_tool_dict("GeoNames",2,4,3,1,8),
make_tool_dict("Transkribus",0,1,2,1,8),
make_tool_dict("Excel",5,9,3,6,7),
make_tool_dict("MySQL",0,6,9,5,7)]
print("\nPrinting tools...\n")
for tool in dh_tools:
for prop in ["name","2015","2019","total"]:
print(prop+": "+str(tool[prop]))
print("")What arguments does the make_tool_dict function take and what value does it return?
What does dh_tools variable contain?
What is the for loop iterating through? What does str(tool[prop]) do?
For more on functions, checkout this tutorial https://www.datacamp.com/community/tutorials/functions-python-tutorial
So far, the code that we've written has flowed forward, line by line. Even the function definition: the contents of the def block are read in order by the interpreter. The order of instructions through which a computer program runs is known as "control flow". We can affect the flow with conditionals and loops which allow us more interesting modes.
Earlier we learned about booleans (True or False). In Python, we can test the truth value of code to decide how we want our code to run.
A conditional is a way for a computer program to make a choice. The basic syntax of the conditional in many programming languages is the if statement. In Python, it looks like this:
x = 5
if x>0:
print("Positive")If statements are essentially expressions that follow if and end with a :. We indent whatever code we want to run if that expression is successful within that code block, just like with functions or for loops. In this case, when whatever value we assign to x is greater than zero our script will run the print statement. What happens if we assign x to -1?
x = 5
if x>0:
print("Positive")
elif x<0:
print("Negative")
else:
print("Zero")We an also use additional conditional keywords elif (else if) and else to add more complexity to our code. Each of the conditional blocks (the three print() statements) are only run if the associated conditional statement is True (in the boolean logic sense). We can have multiple elif blocks if we want. We can also omit elif and else blocks altogether.
We can also test more complicated comparisons using various comparison operators:
For numbers, we can use >, >=, ==, <, and <= to make numeric comparisons. If we want to modify or chain together boolean statements, we can use and, or, and not:
x = 5
y = -1
if x>0 and y<0:
print("both expressions true")For strings, we can use use == for comparison and some special operators like in to see if one string exists inside of another.
if 'I' in 'TEAM':
print('at least one')
else:
print('no')If a variable is the special None object, an empty string (""), or the numeric value zero, it evaluates as boolean False. Otherwise, it is True.
fundamentals = ''
if fundamentals:
print('Yay!')
else:
print('Nooo...')We can also test the equality of two variables in an if statement:
top_tool2015 = 'Gephi'
top_tool2019 = 'Python'
if top_tool2015 != top_tool2019:
print('Change over time')
else:
print('More they stay the same')For more information about control flow, read the Python documentation on the topic.
We'll go over all of this in class with exercises, but bring additional questions too!!