Introduction to Functions
Introduction to Functions
Functions Fundamentals
Introduction
A function is a section of code that performs an action. You create a function by writing code in a Python module (a simple textbased document that contains code). The fundamental formula to create a function is:
def functionname(...): . . .
To create a function, start with the def keyword.
Practical Learning: Starting a Project
loan_amount = 4750 interest_rate = 14.50 # % periods = 42 # Number of months print("Loan Amount: ", loan_amount) print("Interest Rate: ", interest_rate, "%") print("Periods: ", periods, 'months') print("==================================")
The Name of a Function
When creating a function, the def keyword is followed by a name. The name follows the same options you would apply to the name of a variable.
The Parentheses of a Function
The name of a function is followed by parentheses. The parentheses of a function offer many options. To start, the parentheses can be empty. Most of the time, you will write the parentheses on the same line as the name of the function.
The parentheses of a function are followed by a colon (:).
The Body of a Function
The body of a function follows the line after the colon. You will write the code associated with the function starting on the line after the closing of the parentheses and the colon.
Indentation
As seen with conditional statements, indentation consists of pushing a line of code to the right. That indentation is done with regards to the previous line of code. As seen with conditional statements, the code of a function must be indented, usually four characters to the right. This can be illustrated as follows:
def functionname(): continuation
In this case, all the lines of code that belong to the same group are indented at the same level:
def functionname(): line1 line2 linen
In the same way, any line of code that belongs to another dependent line of code must be indented. Here are illustrations:
def functionname(): dependent1level1 dependent2level1 subdependent1level1 subdependent2level1 dependent3level1 subdependent1level1 subdependent2level1 subdependent1level1 subdependent2level1
In some computer languages (like Cbased languages (C#, Java, C++, JavaScript)), indentation of the body of a function (and of a conditional statement) is only used to make code easy to read (which is also the case for languages like Visual Basic, TypeScript, etc). In some other languages like F#, indentation is an integral part of the code. This means that, in those languages, indentation is required. In the same way, in Python, code indentation is required.
When defining a function, you can write each parenthesis on its own line. The opening parenthesis can be written on the first line. If the closing parenthesis is written on the next line, it must be indented. This would be done as follows:
def calculate( ): . . .
If you want, you can write the opening parenthesis on the line that follows the name of the function. In that case, the parenthesis must be indented. Here is an example:
def calculate (): . . .
Or like this:
def calculate ( ): . . .
Defining or Implementing a Function
When you create a function and write its code, you are said to define or implement the function.
Practical Learning: Defining a Function
def process_loan():
loan_amount = 4750
interest_rate = 14.50 # %
periods = 42 # Number of months
print("Loan Amount:", loan_amount)
print("Interest Rate:", interest_rate, "%")
print("Periods:", periods)
print("==================================")
Calling a Function
After creating/defining/implementing a function, you can use it. Using a function is referred to as calling the function. To do this, in the section of code where you need the result of the function, type the name of the function followed by parentheses. Normally, most functions are called from outside their body (the exception is a category of functions named recursive functions: a recursive function is a function that calls itself).
A typical application uses many functions. As a result, you can create as many functions as you need. If you create functions in a document, the compiler will access them in the order they are called, which is usually different from the order the functions were created.
Practical Learning: Calling a Function
def process_loan():
loan_amount = 4750
interest_rate = 14.50 # %
periods = 42 # Number of months
print("Loan Amount:", loan_amount)
print("Interest Rate:", interest_rate, "%")
print("Periods:", periods)
process_loan()
print("==================================")
Loan Amount: 4750 Interest Rate: 14.5 Periods: 42 ================================== Press any key to continue . . .
The Data Type of a Local Variable
If you declare a (some) variable(s) in the body of a function, if you want, you can specify its (their) data type(s).
Practical Learning: Setting Data Types
def process_loan(): loan_amount : float = 18685 interest_rate : float = 9.95 # % periods : int = 60 # Number of months print("Loan Amount:", loan_amount) print("Interest Rate:", interest_rate, "%") print("Periods:", periods) process_loan() print("==================================")
Loan Amount: 18685 Interest Rate: 9.95 % Periods: 60 ================================== Press any key to continue . . .
def process_loan(): print("Watts' A Loan") print("============================================") print("Enter the information to set up the loan") loan_amount = float(input("Loan Amount: ")) interest_rate : float = float(input("Interest Rate: ")) periods : int = int(input("Number of months: ")) iRate : float = interest_rate / 100.00 years : float = periods / 12 interest_amount : float = loan_amount * iRate * years future_value : float = loan_amount + interest_amount print("============================================") print("\tWatts' A Loan") print('') print(f"Loan Amount: {loan_amount}") print(f"Interest Rate: {interest_rate:.2f}%") print(f"Periods: {periods} Months") print('') print(f"Interest Amount: {interest_amount:.2f}") print(f"Future Value: {future_value:.2f}") process_loan() print("===========================================")
Watts' A Loan ============================================ Enter the information to set up the loan Loan Amount: 1850 Interest Rate: 16.85 Number of months: 32 ============================================ Watts' A Loan  Loan Amount: 1850.0 Interest Rate: 16.85% Periods: 32 Months  Interest Amount: 831.27 Future Value: 2681.27 =========================================== Press any key to continue . . .
A Function with a Parameter
Introduction
As stated in our introduction, a function is a section of code that performs a job. In many cases, a function can use only its local values. In some cases, if you want a function to perform a job based on a certain value that doesn't depend on the function itself, you may have to provide that value to the function. To make this possible, when creating the function, type a name for the value in the parentheses of the function. Here is an example:
def display(value):
# . . .
A Parameter
The name you write in the parentheses of a function, when defining the function, is called a parameter. After putting a name in the parentheses of a function, in the body of the function, you can use the parameter anyway you like. For example, you can display the value of that parameter by writing it in the parentheses of print(). Here is an example:
def send_message(msg):
print(msg)
In the same way, you can involve the parameter in an expression.
Calling a Parameterized Function
When you call a function that has a parameter, type the name of the function and its parentheses. In the parentheses, type a value for the parameter. Here is an example:
def send_message(msg): print(msg) send_message("Please remember to submit your time sheet."))
This would produce:
Please remember to submit your time sheet. Press any key to continue . . .
Practical Learning: Creating a Function with Parameter
def process_loan(): print("Watts' A Loan") print("============================================") print("Enter the informaiton to set up the loan") loan_amount = float(input("Loan Amount: ")) interest_rate : float = float(input("Interest Rate: ")) periods : int = int(input("Number of months: ")) iRate : float = interest_rate / 100.00 years : float = periods / 12 interest_amount : float = loan_amount * iRate * years future_value : float = loan_amount + interest_amount print("============================================") print("\tWatts' A Loan") print('') def showPrincipal(a): print(f"Loan Amount: {a}") def showInterestRate(b): print(f"Interest Rate: {b:.2f}%") def showPeriods(c): print(f"Periods: {c} Months") def showAmount(d): print(f"Interest Amount: {d:.2f}") def showFutureValue(e): print(f"Future Value: {e:.2f}") process_loan() print("===========================================")
An Argument
A value you provide in the parentheses of a function is called an argument. When you call a function and provide a value in its parentheses, you are said to pass an argument to the function.
Practical Learning: Passing an Argument to a Function
def process_loan(): print("Watts' A Loan") print("============================================") print("Enter the informaiton to set up the loan") loan_amount = float(input("Loan Amount: ")) interest_rate : float = float(input("Interest Rate: ")) periods : int = int(input("Number of months: ")) iRate : float = interest_rate / 100.00 years : float = periods / 12 interest_amount : float = loan_amount * iRate * years future_value : float = loan_amount + interest_amount print("============================================") print("\tWatts' A Loan") print('') showPrincipal(loan_amount) showInterestRate(interest_rate) showPeriods(periods) print('') showAmount(interest_amount) showFutureValue(future_value) def showPrincipal(a): print(f"Loan Amount: {a}") def showInterestRate(b): print(f"Interest Rate: {b:.2f}%") def showPeriods(c): print(f"Periods: {c} Months") def showAmount(d): print(f"Interest Amount: {d:.2f}") def showFutureValue(e): print(f"Future Value: {e:.2f}") process_loan() print("===========================================")
Watts' A Loan ============================================ Enter the informaiton to set up the loan Loan Amount: 4750 Interest Rate: 18.725 Number of months: 36 ============================================ Watts' A Loan  Loan Amount: 4750.0 Interest Rate: 18.73% Periods: 36 Months  Interest Amount: 2668.31 Future Value: 7418.31 =========================================== Press any key to continue . . .
The Data Type of a Parameter
Because Python is an inferred language, you don't have to specify the data type of a parameter. Still, if you want to assist the compiler with the type of a parameter, in the parentheses of the function, after the name of a parameter, you can type a colon and the data type. Here is an example:
def show_detail(cat : str):
print("Loan Category:")
print(cat)
def evaluate():
loan_amount : float = 4750
interest_rate : float = 14.50 # %
periods : int = 42 # Number of months
print("Loan Amount:")
print(loan_amount)
print("Interest Rate:")
print(interest_rate)
print("Periods:")
print(periods)
show_detail("Personal Loan")
print('')
evaluate()
print("==================================")
Introduction to Functions with many Parameters
Introduction
Just like a function can use one parameter, a function can use more than one parameter. When creating a function, to specify its parameters, in its parentheses, write the name of each parameter and separate them with commas. Here is an example:
def add(a, b):
. . .
In the body of the function, you can ignore all parameters and not use them, you can use one of the parameters and ignore the other parameter(s), or you can use all parameters. Here is an example:
def describe(name, price): print("Item Name: ") printname) print("Marked Price: ") print(price)
Other than that, in the body of the function, you can involve a parameter in any appropriate operation you want.
Practical Learning: Creating a Function With Parameters
def present(a, b, c, d, e): print("============================================") print("\tWatts' A Loan") print('') print(f"Loan Amount: {a}") print(f"Interest Rate: {b:.2f}%") print(f"Periods: {c} Months") print('') print(f"Interest Amount: {d:.2f}") print(f"Future Value: {e:.2f}") def process_loan(): print("Watts' A Loan") print("============================================") print("Enter the informaiton to set up the loan") loan_amount = float(input("Loan Amount: ")) interest_rate : float = float(input("Interest Rate: ")) periods : int = int(input("Number of months: ")) iRate : float = interest_rate / 100.00 years : float = periods / 12 interest_amount : float = loan_amount * iRate * years future_value : float = loan_amount + interest_amount process_loan() print("===========================================")
Calling a Function of many Parameters
After defining a function that uses more than one parameter, you can use it. When calling such a function, write its name and parentheses. In the parentheses, type a value for each parameter, exactly in the order the parameters appear in the function.
Practical Learning: Calling a Function of many Parameters
def present(a, b, c, d, e):
print("============================================")
print("\tWatts' A Loan")
print('')
print(f"Loan Amount: {a}")
print(f"Interest Rate: {b:.2f}%")
print(f"Periods: {c} Months")
print('')
print(f"Interest Amount: {d:.2f}")
print(f"Future Value: {e:.2f}")
def process_loan():
print("Watts' A Loan")
print("============================================")
print("Enter the informaiton to set up the loan")
loan_amount = float(input("Loan Amount: "))
interest_rate : float = float(input("Interest Rate: "))
periods : int = int(input("Number of months: "))
iRate : float = interest_rate / 100.00
years : float = periods / 12
interest_amount : float = loan_amount * iRate * years
future_value : float = loan_amount + interest_amount
present(loan_amount, interest_rate, periods, interest_amount, future_value)
process_loan()
print("===========================================")
Watts' A Loan ============================================ Enter the informaiton to set up the loan Loan Amount: 7450 Interest Rate: 14.645 Number of months: 38 ============================================ Watts' A Loan  Loan Amount: 7450.0 Interest Rate: 14.64% Periods: 38 Months  Interest Amount: 3455.00 Future Value: 10905.00 =========================================== Press any key to continue . . .
Returning a Value from a Function
Introduction
So far, we used the result of each function only in its body. In most cases, you want to use the result of a function outside of that function. In that case, the function must produce a value that other sections of a program can use. When a function produces a result that can be used outside that function, the function is said to return a value.
Returning a Value
After performing one or more calculations in a function, to indicate the value that the function returns, type the return keyword followed by the value you want the function to return. At a minimum, a function can return a constant value, such as a string, a number, or a Boolean value. Here are examples:
def produce_number(): return 0 def get_name(): return 'William Jefferson Clinton'
Practical Learning: Returning a Value from a Function
def show_applicant(fn, ln):
return fn + ' ' + ln
def calculate_interest_amount(amt, rate, per):
iRate = rate / 100.00
loan_in_years = per / 12
interest_amount = amt * iRate * loan_in_years
return interest_amount
def calculate_future_value(amt, rate, per):
iRate = rate / 100.00
loan_in_years = per / 12
interest_amount = calculate_interest_amount(amt, rate, per)
future_value = amt + interest_amount
return future_value
current_value = 3625
interest_rate = 15.85
periods = 38
Calling a Function that Returns a Value
You can call a function that returns a value by simply writing its name and parentheses. Here is an example:
def produce():
print("This is the interior of the function.")
return 0;
produce()
This would produce:
This is the interior of the function. Press any key to continue . . .
In reality, the reason you are returning something from a function is that you want the result of a function to be used, and to be useful, outside the function. As one way to do that, you can declare a variable and assign the function call to it. Here is an example:
def produce():
return 0;
a = produce()
print(a)
This would produce:
0 Press any key to continue . . .
Normally, the reason you declare a variable and assign the function call to it is if you plan to use the returned value more than once. Here is an example:
def produce(): return 68; a = produce() b = a * 12 c = a + 2804 print(a) print("") print(c) print("") print(b)
If you are planning to use the returned value of a function once, you don't need to store the function call in a variable. You can just call the function where its value is needed. Here is an example:
def produce():
return 395;
print(produce())
print("")
In the same way, you can involve the function call to an expression directly where the function call is needed. Here is an example:
def produce():
return 395;
print(produce() + 9574)
print("")
This would produce:
9969  Press any key to continue . . .
Even if you are planning to use the returned value of a function many times, you can still call it only where its value is needed. Here are examples:
def produce(): return 395; print("Value:", produce() + 9574) print("") print(produce(), "times 3 =", produce() * 3) print("") print("Value:", produce() + produce()  552 + produce() * 2) print("==================================")
This would produce:
Value: 9969  395 times 3 = 1185  Value: 1028 ================================== Press any key to continue . . .
Practical Learning: Calling a Function that Returns a Value
def show_applicant(fn, ln):
return fn + ' ' + ln
def calculate_interest_amount(amt, rate, per):
iRate = rate / 100.00
loan_in_years = per / 12
interest_amount = amt * iRate * loan_in_years
return interest_amount
def calculate_future_value(amt, rate, per):
iRate = rate / 100.00
loan_in_years = per / 12
interest_amount = calculate_interest_amount(amt, rate, per)
future_value = amt + interest_amount
return future_value
current_value = 3625
interest_rate = 15.85
periods = 38
iAmt = calculate_interest_amount(current_value, interest_rate, periods)
loan_total = calculate_future_value(current_value, interest_rate, periods)
print("================================")
print("Watts' A Loan")
print("Loan Applicant")
show_applicant("Lynda", "Gibbs")
print('')
print("Loan Details")
print(f"Principal: {current_value:.2f}")
print(f"Interest Rate: {interest_rate:.2f}")
print(f"Periods: {periods}", )
print('')
print(f"Interest Amount: {iAmt:.2f}")
print(f"Future Value: {loan_total:.2f}")
print("================================")
================================ Watts' A Loan Loan Applicant  Loan Details Principal: 3625.00 Interest Rate: 15.85 Periods: 38  Interest Amount: 1819.45 Future Value: 5444.45 ================================ Press any key to continue . . .
Returning an Expression
Most functions are made to perform calculations or some type of processing. If you want a function to return an expression, you can first create the expression, store it in a variable, and then return that variable. Here is an example:
def getFullName():
first = "James"
last = "Naughton"
full = first + " " + last
return full
name = getFullName()
print("Full Name:", name)
print("==================================")
This would produce:
Full Name: James Naughton ================================== Press any key to continue . . .
Otherwise, you can directly return an expression. To do that, create an expression on the right side of return. Here is an example:
def getFullName():
first = "James"
last = "Naughton"
return first + " " + last
name = getFullName()
print("Full Name:", name)
Returning the Result of a Function
You can call a function in the body of another function. Here is an example:
def getFullName(): first = "James" last = "Naughton" return first + " " + last def display(): name = getFullName() print("Full Name:", name) display() print("==================================")
You can also return the value of one function in the body of another function. To do this, you can declare a variable in a function, assign the function call to the variable, and then return that variable from the function. Here is an example:
def getFullName(): first = "James" last = "Naughton" return first + " " + last def process(): name = getFullName() #. . . return name result = process() print("Full Name:", result) print("==================================")
Remember that, usually, the reason you first declare a variable is because you are planning to use that value many times and the reason you are storing the returned value of a function call in a variable is because you are planning to use the returned value many times. If you are planning to use the returned value of the called function once, you can call the function directly where you are returning its value, in which case you call the function on the right side of return. Here is an example:
def getFullName():
first = "James"
last = "Naughton"
return first + " " + last
def process():
return getFullName()
print("Full Name:", process())
print("==================================")
Practical Learning: Returning by Calling a Function
def show_applicant(fn, ln): return fn + ' ' + ln def calculate_interest_amount(amt, rate, per): iRate = rate / 100.00 loan_in_years = per / 12 interest_amount = amt * iRate * loan_in_years return interest_amount def calculate_future_value(amt, rate, per): iRate = rate / 100.00 loan_in_years = per / 12 interest_amount = calculate_interest_amount(amt, rate, per) future_value = amt + interest_amount return future_value current_value = 24886 interest_rate = 11.625 periods = 60 print("================================") print("Watts' A Loan") print("Loan Applicant") show_applicant("Lynda", "Gibbs") print('') print("Loan Details") print(f"Principal: {current_value:.2f}") print(f"Interest Rate: {interest_rate:.2f}") print(f"Periods: {periods}", ) print('') print(f"Interest Amount: {calculate_interest_amount(current_value, interest_rate, periods):.2f}") print(f"Future Value: {calculate_future_value(current_value, interest_rate, periods):.2f}") print("================================")
================================ Watts' A Loan Loan Applicant  Loan Details Principal: 24886.00 Interest Rate: 11.62 Periods: 60  Interest Amount: 14464.99 Future Value: 39350.99 ================================ Press any key to continue . . .


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