Raspberry Pi - Lecture 5 Python for Raspberry Pi

Prepared by: Mohamed AbdAllah
Raspberry pi interfacing
Lecture 5: Python for raspberry pi
1

Python Agenda
 Python syntax.
 Strings and console output.
 Conditional and control flow.
 Functions.
 Lists and dictionaries.
 Loops.
 Bitwise operators.
 Classes.
 File Input/output
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 #!/usr/bin/python
• It should be written at the start of any python script to tell the shell to
execute the script using python, so the script should have execute
permission.
 print “Hello”
• Prints Hello to console
 x = 3
• Define integer variable x with value 3
 y = 4.5
• Define float variable y with value 4.5
 z = True
• Define Boolean variable z with value True
 x = 5/2
• Value of x will be 2 as operation is all on integers
Python syntax
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 x = 5.0/2
• Value of x will be 2.5
 x = float(5)/2
• Value of x will be 2.5
 # comment
• # is used to comment out any line
 “””Comment multiple lines”””
• “”” any number of lines “”” is used to comment out any number of
lines.
Python syntax
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 x = 10 + 20
• X is sum of 10 and 20
 x = 4 ** 2
• X is 4 to the power of 2
 x = 101 % 2
• X is 101 modulo 2 which will equal 1
Python syntax
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 var = “Hello Python”
• Put string “Hello Python” in variable var
 c = “Hello”[0]
• Put character at index zero of string “Hello” which is ‘H’ in variable c, so
c will contain the value ‘H’
 c = var[4]
• c will have the value at index 4 which is ‘o’
 len(var)
• Returns the length of the given string
 d = str(32)
• d will contain the string representation of the value 32, so d will contain
“32” as a string.
Strings and console output
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 print var.lower()
• Print string in variable var with all letters lower case.
 print var.upper()
• Print string in variable var with all letters upper case.
 mystr = var + “Scripting”
• Concatenate strings, value of mystr will be “Hello Python Scripting”
 print “Welcome” + “ to” + “ python”
• Prints Welcome to python
 mystr = var[2:len(var)]
• Variable mystr will contain string inside var from index 2 (third
character) till the end of the string inside var.
 Print “value1 is %s, value2 is %s” %(var1, var2)
• Prints the values inside variables var1 and var2.
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 Date and time
# import is used to include external library to be used inside our script
from datetime import datetime
now = datetime.now()
print now.year
print now.month
print now.day
print '%s/%s/%s' % (now.month, now.day, now.year)
print ("%s:%s:%s") % (now.hour, now.minute, now.second)
print ("%s/%s/%s %s:%s:%s") % (now.month, now.day, now.year, now.hour,
now.minute, now.second)
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Conditional and control flow
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 Comparators > == <= <
• Compare between two values, returns True or False
 and, or operators
• Combine between 2 or more conditions
 not operator
• Returns the inverse of the condition
• not is evaluated first, and is evaluated next, or is evaluated last. So use
Parentheses better
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 If syntax
if x>10 : #note the ‘:’
print “Greater” # note the indentation (4 white spaces)
elif x<10 :
print “Less”
else :
print “Equal”
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 Take input from user
name = raw_input("What's your name?") # returns string
integer = int(raw_input("Enter number: ")) # convert returned string to integer
integer = input("enter integer") # returns integer
if name.isalpha() # check that name contains alphabetical characters only
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 Python functions
import math
print math.sqrt(25) # print square root of 25
from math import sqrt # to be able to use sqrt by its name
print sqrt(25)
import math # Imports the math module
everything = dir(math) # Sets everything to a list of function from math
print everything # Prints all available functions in math module
Functions
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 Python functions
maximum = max(-2,3,1,15) # return maximum of given values
minimum = min(3,7,2,9) # return maximum of given values
absolute = abs(-42) # return absolute value of given number
print type(3) # prints <type 'int'>
print type(2.4) # prints <type 'float'>
print type("Hello") # prints <type 'str'>
#no need to import any module to be able to use previous functions as
they are built in functions.
Functions
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 User defined functions
def print_positive(arg) : # take one argument
if agr>=0 :
print arg
return True # return value to the caller
else :
return False
Functions
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 list = [item1, item2, item3]
• Create list of the given 3 items
 list.append(newItem)
• Add new element at the end of the list
 list[0:3]
• Values of the list from index 0 and stop before index 3
 list[:2]
• Grabs the first two items
 list[3:]
• Grabs the fourth through last items
 string_index = list.index(“mystring")
• Find index of “mystring"
Lists and dictionaries
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 list.insert(index, item)
• insert item at index and shift all next items down by 1
 list.sort()
• sort list ascending
 list.remove(item3)
• remove item3 from list
 Loop on all list items
my_list = [1,9,3,8,5,7]
for number in my_list:
print 2 * number # print 2 18 6 16 10 14
22

 students= {‘std1' : 90, ‘std2' : 85, ‘std2' : 93}
• Assigning a dictionary with three key-value pairs to students
 print students[‘std1']
• Prints std1 grade which is 90
 dict_name[new_key] = new_value
• add new value to dictionary
 del dict_name[key_name]
• delete key-value pair from dictionary
 dict_name[key] = new_value
• change value
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a = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]
# print even numbers in list
for number in a :
if number%2 == 0 :
print number
# loop on string characters
for letter in “Mystring":
print letter # print all letters each one on separate line
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# function take list as argument
def count_small(numbers): # returns number of values less than 10
total = 0
for n in numbers:
if n < 10:
total = total + 1
return total
num = [4, 8, 15, 16, 23, 42]
small = count_small(num)
print small # print number of values in list that are less than 10
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# shop example
shopping_list = ["banana", "orange", "apple"]
stock = {"banana": 6, "apple": 0, "orange": 32}
prices = {"banana": 4, "apple": 2, "orange": 1.5}
def compute_bill(food) : # compute the total price of given food list
total = 0
for item in food : # loop on all items in food list
if stock[item]>0 : # if item exists in stock
stock[item]-=1 # reduce stock by 1
total+=prices[item] # add item price to total
return total
Print compute_bill(shopping_list)
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n = [1, 3, 5]
n.pop(1)
# Returns 3 (the item at index 1) and remove it from list
n.remove(1)
# Removes 1 from the list,
# NOT the item at index 1
del(n[1])
# Doesn't return anything, but removes item at index 1
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def my_function(x):
for i in range(0, len(x)): # loop on all indices in list x
x[i] = x[i] * 2 # change value at index i with the double of this value
return x # return the edited list
 Note
range(6) # [0,1,2,3,4,5]
range(1,6) # [1,2,3,4,5]
range(1,6,3) # [1,4] from 1 to 6 with step 3
letters = ['a', 'b', 'c', 'd']
print " ".join(letters) # prints a b c d
print "---".join(letters) # prints a---b---c---d
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# list comprehension
evens_to_50 = [i for i in range(51) if i % 2 == 0] # list of even numbers till
50
my_list[::2] # from start to end with stride of 2
my_list[::-1] # from end to start with stride of 1
squares = [x**2 for x in range(5)] # contains squares of numbers from 0
to 4
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 Note
# to use random numbers
from random import randint
coin = randint(0, 1)
dice = randint(1, 6)
Loops
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count = 0
while count < 10 : # Add a colon
print count
# Increment count
count +=1
# or
while True :
print count
count +=1
if count >= 10:
break #### using break
Loops
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count = 0
count = 0
while count < 3:
num = random.randint(1, 6)
print num
if num == 5:
print "Sorry, you lose!"
break
count += 1
else:
print "You win!" ## will be executed only if didn't enter loop, or loop
terminated normally, but it will not be executed if loop terminated by
"break" statement, the same like "for/else"
Loops
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choices = ['pizza', 'pasta', 'salad', 'nachos']
print 'Your choices are:'
for index, item in enumerate(choices):
print index+1, item # >> Your choices are: 1 pizza 2 pasta 3 salad 4
nachos
Loops
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print 0b11 # print 3
print bin(2) # print bin() returns binary representation of a number
(similar oct() hex())
#note you can't use the return as number any more
print int("0b11001001", 2) # print the integer base ten of the given binary
number
Bitwise operators
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 shift_right
0b1001 >> 1 # 0b0100
 shift_left
0b1001 << 1 # 0b0010
0b1110 & 0b0101 # 0b0100
0b1110 | 0b0101 # 0b1111
0b1110 ^ 0b0101 # 0b1011
print ~1 # print -2
print ~2 # -3
print ~3 # -4
Bitwise operators
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# check value of bit 4
def check_bit4(input):
mask = 0b1000
result = input & mask # using mask
if result > 0:
return "on"
else:
return "off"
Bitwise operators
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a = 0b10111011
print bin(a|0b100) # set on bit3
print bin(a^0b11111111) # flip all bits in a
(0b1 << n-1) ^ number # much simpler to flip bit number n
Bitwise operators
39

 Class: is an object oriented programing concept, class means a
collection of some attributes (variables) and methods (functions) that
are related to the same physical meaning.
 Defining a class: means defining a new collection of attributes and
methods that are related to each other to be used later.
 Using class: is done by making a variable (instance) of that class, one
can make multiple instances of the same class and they are all
independent.
Classes
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# define new class
class Animal(object):
# attributes section definition
is_alive = True
health = "good“
# init method, called automatically each time an instance is made of
the class
def __init__(self, name, age): #all methods must take (self) as first as
this is the variable that points to the current instance
self.name = name
self.age = age
# any methods are added here
def description(self):
print self.name
print self.age
Classes
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# using class
hippo = Animal("any1", 3) # make new instance
hippo.description() # use the method for hippo instance
sloth = Animal("any2", 3) # make new different instance
ocelot = Animal("any3", 4) # make new different instance
# they all will print “good” as no one changed the initial value
print hippo.health
print sloth.health
print ocelot.health
Classes
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# Read from file
my_file = open("output.txt", "r") # open file with read only permission
print my_file.read() # read file content and print it
my_file.close() # close file
# Write to file
my_file = open("output.txt", “r+") # open file with read / write
permission
my_file.write(“Hello”) # write hello to file
my_file.close() # close file
File Input/output
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# make python close the file by itself after finishing
with open("text.txt", "w") as textfile:
textfile.write("Success!")
if my_file.closed == False: # check if file not closed through closed
attribute
my_file.close()
print my_file.closed # print closed attribute current value, it
should now be True as the file is closed
File Input/output
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 Different access modes:
r Opens a file for reading only. The file pointer is placed at the
beginning of the file. This is the default mode.
r+ Opens a file for both reading and writing. The file pointer placed at
the beginning of the file.
w Opens a file for writing only. Overwrites the file if the file exists. If the
file does not exist, creates a new file for writing.
w+ Opens a file for both writing and reading. Overwrites the existing file if
the file exists. If the file does not exist, creates a new file for reading and
writing.
File Input/output
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 Different access modes:
a Opens a file for appending. The file pointer is at the end of the file if
the file exists. That is, the file is in the append mode. If the file does not
exist, it creates a new file for writing.
a+ Opens a file for both appending and reading. The file pointer is at the
end of the file if the file exists. The file opens in the append mode. If the
file does not exist, it creates a new file for reading and writing.
File Input/output
48

Mohamed AbdAllah
Embedded Systems Engineer
mohabdallah8@gmail.com
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Raspberry Pi - Lecture 5 Python for Raspberry Pi

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