Lesson 3 - Operations

Addition

>>> 1 + 2
3

Subtraction

>>> 1 - 2
-1

Multiplication

>>> 1 * 2
2

Division

>>> 4 / 2
2

In Python 2, division of two integers is an integer

>>> 3/2
1

In Python 3, division of two integers is a float

>>> 3/2
1.5

To get a float result in Python 2, make one number a float

>>> float(3) / 2
1.5

To get Python 3 behavior in Python 2

from __future__ import division

To return only whole numbers, use // (floor division)

>>> 1 // float(2)
0.0
>>> float(3) // 2
1.0

Modulus - Return the remainder

>>> 1 % 2
1
>>> 7 % 5
2

Return whole number and remainder

>>> divmod(1, 2)
(0, 1)
>>> divmod(7, 5)
(1, 2)

Exponents

>>> 2 ** 3
8
>>> pow(2, 3)
8

Absolute Value

>>> abs(-1)
1
>>> abs(2j)
2.0

To do more complex math see the math module

To do really complex math:

• NumPy
• SciPy

Python is PEMDAS compliant

• P - Parentheses
• E - Exponents
• M - Multiplication
• D - Division
• A - Addition
• S - Subtraction

>>> 2 ** 2 + 10 / 2
9
>>> 2 ** (2 + 10) / 2
2048

Functions and lookups are completed before operations

>>> 2 * abs(-2)
4

Equal (==)

>>> 1 == 1
True
>>> "morning" == "night"
False

Not Equal (!=)

>>> 1 != 1
False
>>> "morning" != "night"
True

Less than (<)

>>> 1 < 2
True

Less than or equal (<=)

>>> 2 <= 2
True

Greater than (>)

>>> 2 > 1
True

Greater than or equal (>=)

>>> 2 >= 2
True

Identity (is / is not)

>>> 1 is 1
True
>>> [1, 2] is [1, 2]
False

>>> 1 is not 1
False
>>> [1, 2] is not [1, 2]
True

• is tests if two objects are the same, not if they have the same value

OR

>>> False or False
False
>>> False or True
True

AND

>>> True and True
True
>>> True and False
False

NOT

>>> not True
False
>>> not False
True

Union - Create new set with elements from all sets

>>> set([1, 2]) | set([2, 3]) | set([2, 3, 4])
set([1, 2, 3, 4])

Intersection - Create new set with elements common to all sets

>>> set([1, 2]) & set([2, 3]) & set([2, 3, 4])
set([2])

Difference - New set with elements appearing only in first set

>>> set([1, 2]) - set([2, 3, 4])
set([1])

Symmetric Difference - New set with unique elements from each set

>>> set([1, 2]) ^ set([2, 3, 4])
set([1, 3, 4])

Subset - True if every element in first set is in second set

>>> set([1, 2, 3]) <= set([1, 2, 3, 4])
True

Proper Subset - True if subset and sets are not equal

>>> set([1, 2, 3, 4]) < set([1, 2, 3, 4])
False

Superset - True if every element in second set is in first set

>>> set([1, 2, 3, 4]) >= set([1, 2, 3])
True

Proper Superset - True if superset and not equal

>>> set([1, 2, 3, 4]) > set([1, 2, 3, 4])
False

The basic assignment operator is the equal sign

>>> var1 = 2

There is an assignment shortcut for basic math operators

>>> var1 = 2
>>> var1 += 1
>>> var1
3
>>> var2 = 7
>>> var2 %= 5
>>> var2
2

For sequences, concatenation and assignment can be combined

>>> mylist = [1, 2]
>>> mylist += [3, 4]
>>> mylist
[1, 2, 3, 4]

Works for set operations too

>>> mySet = set([1, 2, 3])
>>> mySet ^= set([2, 3, 4])
>>> mySet
set([1, 4])

For example, the str and repr output for an exception is different

>>> e = RuntimeError("Something went wrong")
>>> repr(e)
"RuntimeError('Something went wrong',)"
>>> str(e)
'Something went wrong'

In Python 3, the ascii() function returns the repr() output with non-ASCII characters escaped

The % operator can be used to format strings

The format is similar to sprintf() in C

Values can be specified in a tuple or dictionary

>>> "Hi! My name is %s. I can count to %d" % ('George', 732)
'Hi! My name is George. I can count to 732'

>>> "Hi! My name is %(name)s. I can count to %(number)d" % \
... {'name' : 'George', 'number' : 732}
'Hi! My name is George. I can count to 732'

Single values can be on their own

>>> 'Hello, %s' % 'world'
'Hello, world'

Pad integer with zeros

>>> '%03d' % 12
'012'

Float precision to 3 decimals

>>> '%.3f' % 12.1236
'12.124'

Float precision as a variable

>>> '%.*f' % (3, 12.1236)
'12.124'

Left pad string to 15 characters

>>> '%15s' % 'hello'
'          hello'

Right pad string to 15 characters

>>> '%-15s' % 'hello'
'hello          '

Print representation string for an object

>>> '%r' % Exception("something bad Happened")
"Exception('something bad Happened',)"

In Python 2.6, the str.format() method was introduced

Preferred method in newer versions of Python

More flexible than the old format

By default, objects are converted using their own __format__() method

Falls back to str() and then repr()

>>> # In Python 2.7+, no name or index is required
... "Hi! My name is {}. I can count to {}".format('George', 732)
'Hi! My name is George. I can count to 732'

>>> "Hi! My name is {1}. I can count to {0}".format(732, 'George')
'Hi! My name is George. I can count to 732'

>>> "Hi! My name is {name}. I can count to {number}".format(
... name='George', number=732)
'Hi! My name is George. I can count to 732'

Pad integer with zeros

>>> '{:03d}'.format(12)
'012'

Float precision to 3 decimals

>>> '{:.3f}'.format(12.1236)
'12.124'

Float precision as a variable

>>> '{1:.{0}f}'.format(3, 12.1236)
'12.124'

Left pad string to 15 characters

>>> '{:>15}'.format('hello')
'          hello'

Right pad string to 15 characters

>>> '{:<15}'.format('hello')
'hello

Print representation string for an object

>>> '{!r}'.format(Exception("something bad Happened"))
"Exception('something bad Happened',)"

Center within 15 characters with underscores

>>> '{:_^15}'.format('hello')
'_____hello_____'

Print as a percentage to two decimal places

>>> '{:.2%}'.format(0.56)
'56.00%'

Perform multiple substitutions with the same value

>>> '{0}-di, {0}-da'.format('ob-la')
'ob-la-di, ob-la-da'

Introduced in Python 3.6

Allow expressions to be embedded in strings

Faster than other formatting options

No backslashes in expressions (OK in string)

No comments in expressions

Curly braces are escaped with more curly braces

>>> name = 'George'
>>> limit = 732
>>> f"Hi! My name is {name}. I can count to {max(10, limit)}"
'Hi! My name is George. I can count to 732'

Conversion - same as str.format()

• !s – Convert using str()
• !r – Convert using repr()
• !a – Convert using ascii() (Python 3 Only)

The format specification (formatSpec) same as str.format()

• Can be object-specific

• Most objects use a common language

  • See datetime.datetime for an object with special formatting

Raw f strings can be created by prepending fr

>>> interpreted = 'Nope'
>>> fr'I\'m not interpreted? {interpreted}'
"I\\'m not interpreted? Nope"

Basic slice

>>> myList = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> myList[2:5]
[2, 3, 4]

>>> myList[4:]
[4, 5, 6, 7, 8, 9]

>>> myList[-3:]
[7, 8, 9]

>>> myList[:4]
[0, 1, 2, 3]

>>> myList[:]  # Creates a shallow copy of a list
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Extended Slice

>>> myList = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> myList[2:8:2]
[2, 4, 6]
>>> myList[::-3]
[9, 6, 3, 0]

zip() creates a list of tuples with an element from each iterable

• In Python 3, an iterator of tuples is returned instead of a list
• Result is only as long as the shortest iterable

Syntax: zip([iterable, …])

>>> input1 = [1, 2 , 3, 4, 5, 6]
>>> input2 = [2, 4, 6, 8, 10]
>>> zip(input1, input2)
[(1, 2), (2, 4), (3, 6), (4, 8), (5, 10)]

Some objects like, dictionaries, have a copy() method

• Shallow copy method, does not make copies of the contents

>>> myDict = {'aList' : [1, 2]}
>>> newDict = myDict.copy()
>>> newDict['aList'].append(3)
>>> myDict['aList']
[1, 2, 3]

To copy an object and it’s contents, use deepcopy() from the copy module

>>> import copy
>>> myDict = {'aList' : [1, 2]}
>>> newDict = copy.deepcopy(myDict)
>>> newDict['aList'].append(3)
>>> myDict['aList']
[1, 2]