Branching in Python
Consider we are in a class with the following students defined in a dictionary together with their roles in the class.
class_dict = {
, 'Christian Chrysler' : 'student'
, 'Matthew Bagby' : 'student'
, 'Niyousha Dredger' : 'student'
, 'Pauline Davachi' : 'student'
, 'Marcos Kuffel' : 'student'
, 'Lauren Kumbhare' : 'student'
, 'Shashank Guillen' : 'student'
, 'Hany Moorman' : 'student'
, 'Sarah Mahdy' : 'student'
, 'Andrew Osborne' : 'student'
, 'Joshua Myers' : 'student'
, 'Rebecca Vedovato' : 'student'
, 'Carolina Proni' : 'student'
, 'Amir Shahmoradi' : 'instructor'
}
Now suppose we would like to write a Python script that, when executed, asks the user to input the full name of a person in our class and then outputs on screen, the role of the person in class. To do this, first we should get familiar with Python’s built-in function input().
raw_input(). In Python 3, it is now renamed to input().input("\n Please enter the full name of the person: ")
Please enter the full name of the person: Amir Shahmoradi
'Amir Shahmoradi'
The function input() is a Python built-in function that outputs the input string inside parentheses to the output screen, and then waits for the user to enter an input. This function reads a line from input, and converts it to a string (stripping a trailing newline), and returns that. One can also put the user’s input directly into a variable, which is the normal way of using this function.
input_variable = input("\n Please enter the full name of the person: ")
Please enter the full name of the person: Amir Shahmoradi
print(input<press tab>
input input_key input_variable
print(input_variable)
Amir Shahmoradi
type(input_variable)
str
input_variable = input("\n Please enter the full name of the person: ")
Please enter the full name of the person: 1234
type(input_variable) # whatever type the input is, it will be converted to a string by Python interpreter
str
Now, back to our originally stated problem in the above, we want to write a program that takes in the name of a person from the command line and then tells the user some information about them, depending on their name. To achieve this, we need to become familiar with the concept of branching in Python. Specifically, we can achieve our goal by writing an if-elif statement in Python as in the following python script.
The general syntax for if blocks in python is the following.
if expression1: # if expression1 is True
statement(s) # execute the required Python statements
elif expression2: # else if expression2 is True
statement(s) # execute the required Python statements
elif expression3: # else if expression3 is True
statement(s) # execute the required Python statements
else: # else if neither of the above was True
statement(s)
Note that the indentations at the beginning of each of the statements are necessary, otherwise the Python interpreter will give you a syntax error. However, it is important to note also that the number of indentations is arbitrary.
tab character can create a huge mess in your Python codes, and some extra work to clean them up. Here is a good resource to learn more about indentations in Python.#!/usr/bin/env python
class_dict = { 'nicholas dhana' : 'student'
, 'bradley driver' : 'student'
, 'sagar boeker' : 'student'
, 'travis bridges' : 'student'
, 'eric garcia' : 'student'
, 'christian gagliano' : 'student'
, 'matthew janssen' : 'student'
, 'lucero goree' : 'student'
, 'jake herrera' : 'student'
, 'michael lewis' : 'student'
, 'colin langford' : 'student'
, 'mark mendiola' : 'student'
, 'emilio loveland' : 'student'
, 'kreshel philley' : 'student'
, 'russell nguyen' : 'student'
, 'caleb robbins' : 'student'
, 'joseph phillips' : 'student'
, 'bradley varier' : 'student'
, 'vivek smith' : 'assistant'
, 'amir shahmoradi' : 'instructor'
}
name = input("\n Please enter the full name of the person: ")
if name in class_dict: # First make sure the name is in our dictionary
if class_dict[name] == 'instructor':
print( '\nThe name you enetered: {} belongs to the instructor of this course. His office hours are Mondays 3-4 p.m.\n'.format(name) )
elif class_dict[name] == 'assistant':
print( '\nThe name you enetered: {} belongs to the Teaching Assistant of this course.\n'.format(name) )
elif class_dict[name] == 'student':
print( '\nThe name you enetered: {} belongs to one of the students of our class.\n'.format(name) )
else:
print('\nThe name you entered: {} does not correspond to any real person in this class. Make sure you are not looking for a ghost, as our class is ghost-free.\n'.format(name) )
Now, if we run the file containing this script, we will get something like the following,
$ ./class_roster.py
Please enter the full name of the person: amir shahmoradi
The name you entered: amir shahmoradi belongs to the instructor of this course. His office hours are on Mondays at 5-6 p.m.
$ ./class_roster.py
Please enter the full name of the person: Harry Potter
The name you entered: Harry Potter does not correspond to any real person in our class. Make sure you are not looking for a ghost, as our class is ghost-free.
Conditional if statement is the only built-in branching method in Python. However, it can be written in several different sytaxes, each of which can be useful in some circumstances:
1. One-line conditional statement:
if condition: statement
For example:
if sqrt(2) < 2: print('sqrt(2) < 2\nOf course that was obvious!')
sqrt(2) < 2
Of course, that was obvious!
2. multiple line (as stated above) conditional statement:
if condition:
block statements
elif:
block statements
else:
block statements
3. Inline conditional expression: This is a particularly useful syntax for conditional value assignments in Python.
expression1 if condition_is_met else expression2
For example, instead of writing,
if condition:
a = value1
else
a = value2
one can summarize it all in one line,
a = (value1 if condition else value2)
Note that the paratheses are not necessary, however, they are recommended for clarity. Here is an example code,
name = input('Input the name: ')
print( 'This person is the course instructor' if name == 'amir' else 'This person is not the course instructor')
Input the name: amir
This person is the course instructor
Non-boolean conditions in if-statements
There is a rather interesting feature of conditions in Python if-statements, that allows the programmer to use a non-boolean variable or value type directly in place of the condition in if-statement. What really happens here is that Python interpreter converts the non-boolean type to a boolean value, when it occurs in place of an if-statement condition.
if 5.6:
print('The condition in this if-statement is automatically converted from float to boolean')
The condition in this if-statement is automatically converted from float to boolean
if 0.0:
print('A float value of zero is converted to False')
if not 0.0:
print('A float value of zero is converted to False')
A float value of zero is converted to False
if 0.000000000000000000000000000000000000000000000000000000000000000000001:
print('Any non-zero float value of any precision is converted to True')
A float value of zero is converted to False
if 1.e-323:
print('Any non-zero float value of any precision is converted to True')
Any non-zero float value of any precision is converted to True
if 1.e-324: # make sure you don't go beyond computer precision
print('Any non-zero float value smaller than the computer precision will be set to 0')
if not 1.e-324: # make sure you don't go beyond computer precision
print('Any non-zero float value smaller than the computer precision will be set to 0')
Any non-zero float value smaller than the computer precision will be set to 0
if 12:
print('The same rules also hold for integers.')
The same rules also hold for integers.
if "":
print('An empty string is converted to boolean False')
if not "":
print('An empty string is converted to boolean False')
An empty string is converted to boolean False
if " ":
print('A non-empty string is converted to boolean True')
A non-empty string is converted to boolean True
if []:
print('An empty list is converted to boolean False')
if not []:
print('An empty list is converted to boolean False')
An empty list is converted to boolean False
if [1]:
print('A non-empty list is converted to boolean True')
A non-empty list is converted to boolean True
if not {}:
print('The same rules also hold for sets and dictionaries.')
The same rules also hold for sets and dictionaries.
if {1:2}:
print('The same rules also hold for sets and dictionaries.')
The same rules also hold for sets and dictionaries.
if not None:
print('The keyword None is also equivalent to False.')
The keyword None is also equivalent to False.
bool("amir") # You can always get the boolean-conversion of a given value or type using Python's built-in function bool().
True
Exercise
- Single-line Python input and string manipulation: web-link