Sunday, March 26, 2017

Python Part5: Set and exception handling Try except finally

Set

Unordered collection of unique, immutable objects.
Literals:

delimited by { and }

Single comma separated items.

>>> p = {1, 2, 3, 4}
>>> p
{1, 2, 3, 4}

Empty { } makes a dict, so for empty set use the set() constructor.

>>> e = set()
>>> e
set()

Set() constructor accepts:

Iterable series of values.

>>> s = set([1, 2, 3, 4])
>>> s
{1, 2, 3, 4}

Duplicates are discarded.

>>> t = [1, 2, 3, 1, 4]
>>> set(t)
{1, 2, 3, 4}

Often used specifically to remove duplicates – not order preserving.
Order is arbitrary.

>>> for x in {1, 2, 4, 8, 32}
print(x)

Use in and not in operators.

>>> q = {2, 9, 6, 4}
>>> 3 in q
False
>>> 3 not in q
True

Add(item) inserts a single element.

>>> k = {2, 4}
>>> k
{2, 4}
>>> k.add(8)
>>> k
{2, 4, 8}

Duplicates are silently ignored.

>>> k.add(8)
>>> k
{1, 2, 8}

For multiple elements use update(items) passing any iterable series.

>>> k.update([9, 10])
>>> k
{1, 2, 8, 9, 10}

Remove(item) requires that item is present, otherwise raises KeyError.

>>> k.remove(9)
>>> k
{1, 2, 8, 10}

Discard(item) always succeeds.

>>> k.discard(9)

Copy set using S.copy() method.

>>> j = k.copy()

Use constructor. Set(s)

>>> m = set(j)

Copies are shallows.
S1.union(s2) method.

Union is commutative.

s.intersection(t) method

Intersection is commutative.

s.difference(t) method.

Difference is non-commutative.

s.symmetric_difference(t) method

Symmetric difference is commutative.

s.issubset(t) method
s.issuperset(t) method.
s.isdisjoint(t)

Handle exceptions. Try … except … finally

try:

x = 5

except:

message = str(sys.exc_info()[1])
x = -1

finally:

# final-block
x = 0

Python Part4: For loop, argument passing and Tuple

For loop

items = [a, b, c, d]
for item in items:

print(item)

for i in range(5)

print(i)

Argument passing

Function with arguments.
def modify(k)

k.append(13)
print(k)

m = [6, 7, 8, 9]
modify(m)
Default arguments.
def DisplayMessage(m='nothing')

print(m)

message = 'Welcome to python'
DisplayMessage(Message)
DisplayMessage()

Tuple

Heterogeneous immutable collection.
Delimited by parentheses.

For example t = ("norway", 4.953, 3)

Items separated by commas.
Element access with square brackets and zero-based index. T[index]

>>> t[0]
'Norway'

len(t) for number of elements.

>>> len(t)
3

Iteration with for loop.

for item in t:
print(item)

Norway
4.953
3

Concatenation with + operator.

t + (338186.0, 2659)
('Norway', 4.953, 3, 338186.0, 2659)

Reputation with * operator.

t * 3
('Norway', 4.953, 3, 'Norway', 4.953, 3, 'Norway', 4.953, 3)

Tuple can contain any kind of object.
Nested tuples.

A = ((245, 446), (90, 456))

Chain square-brackets indexing to access inner elements.

A[1][0]
90

Can’t use one object in parentheses as a single element tuple.
For a single element tuple include a trailing comma.

K = (234, )

The empty tuple is simply empty parentheses.

E = ()

Delimiting parentheses are optional for one or more elements.

p = 1, 1, 1, 4, 6, 19
>>> p
(1, 1, 1, 4, 6, 19)

Tuples are useful for multiple return values.

def MinMax(items)

Return Min(items), Max(items)

>>> MinMax([33, 30, 9, 82, 87])
(9, 87)

Tuple unpacking allow us to destructure directly into named references.

lower, upper = MinMax([33, 30, 9, 82, 87])
>>> lower
9
>>> upper
87

Tuple unpacking works with arbitrarily nested tuples (although not with other data structure)

(a, (b, (c, d))) = (4, (3, (2, 1)))
>>> a
4
>>> b
3
>>> c
2
>>> d
1

a, b = b, a is the idiomatic python swap.
Use the tuple constructor to create tuples from other iterable series of objects.

>>> tuple([234, 444, 897])
(234, 444, 897)
>>> tuple("car")
('c', 'a', 'r')

The in and not in operators can be used with tuples - and other collection types – for membership testing.

>>> 5 in (3, 5, 9, 13)
True
>>> 5 not in (3, 5, 9, 13)
False

Saturday, March 25, 2017

Python Part3: Bytes, Lists, Shallow copies and Dictionaries

Bytes

Immutable sequence of bytes.

Lists

Mutable sequences of objects.

e.g a = [1, 2, 3 ,4]

Get or set value using index

e.g a[1] = 20 and print(a[1])

Negative integers index from the end.
The last element is at index -1
Slicing extracts part of a list.

Slice = seq[start:stop]

Slice range is half open-stop not included.

>>> s = [1, 2, 3, 4, 5, 6]
>>> s[1:4]
2, 3, 4

Slicing works with negative indexes.

>>> s[1:-1]
2, 3, 4, 5

Omitting the stop index slices to the end.

Slice_to_end = seq[start:]

Omitting the start index slices from the beginning.

Slice_from_beginning = seq[:stop]

Half open ranges give complementary.

slices S[:x] + s[x:] == s

Omitting the start and stop indexes slices from beginning to the end – a full slice.

Full_slice = seq[:]

Important idiom for copying lists:
Copy list using

Copy() method

U = seq.copy()

List() constructor

V = list(seq)

All of these techniques create shallow copies, mean create new list containing same object reference to source list.

Shallow copies: Examples

>>> A = [[1, 2], [3, 4]]
>>> B = A[:] (copy using full slice)
>>> A is B (both objects have different identities.)

False

>>> A == B

True

>>> A[0]

[1, 2]

>>> B[0]

[1, 2]

A[0] is B[0] (both objects have same identities.)

True

>>> A[0] = [8, 9] (This create a new list and set reference of A[0] to this.)
>>> A[0]

[8, 9]

>>> B[0] (B[0] is unchanged because it has reference to old list.)

[1, 2]

>>> A[1].append(5) (A[1] and B[1] still have reference to same list. So changes reflects in both places.)
>>> A[1]

[3, 4, 5]

>>> B[1]

[3, 4, 5]

>>> A (Now both lists look like this)

[[8, 9], [3, 4, 5]]

>>> B

[[1, 2], [3, 4, 5]]

Repeat lists using the * operator.
Most often used for initializing a list of known size with a constant.

S = [constant] * size

Multiple references to one instance of the constant in the produced list.
Repetition is shallow.

>>> S = [[1, 2]] * 5

[[1, 2], [1, 2], [1, 2], [1, 2], [1, 2]]

>>> S[3].append(7)
>>> S

[[1, 2, 7], [1, 2, 7], [1, 2, 7], [1, 2, 7], [1, 2, 7]]

Index(item) returns the integer index of the first equivalent element raises value error if not found.

I = w.index(‘the’)

Count(item) returns the number of matching elements.

W.count(‘the’)

The in and not in operators test for membership.

5 in [1, 2, 3]
5 not in [1, 2, 3]

Del seq[index] to remove by index.
Seq.remove(item) to remove by value. Raises value error if not found.
Insert items with seq.insert(index, item)
Concatenate lists with + operator

>>> M = [1, 2]
>>> N = [3, 4]
>>> K = m + n
>>> k

[1, 2, 3, 4]

In-place extension with += operator or extend method.

>>> K += [6, 7]
>>> K

[1, 2, 3, 4, 6, 7]

>>> K.entend([11, 12])
>>> K

[1, 2, 3, 4, 6, 7, 11, 12]

K.reverse() reverses in place.
K.sort() sorts in place.
K.sort(reverse=true) gives descending sort.

Dictionaries

Unordered mapping from unique, immutable keys to mutable values.
Recap literals:

Delimited by { and }
Key-value pairs comma separated
Corresponding keys and values joined by colon.
Keys must be unique
Urls = {'google':'http://google.com', 'microsoft':'http://Microsoft.com' }
>>> url['google']

http://google.com

Dict() constructor accepts:

Iterable series of key-value 2-tuples.

>>> Names_and_ages = [('Alice', 32), ('Bob', 48)]
>>> d = dict(Names_and_ages)
>>> d

{'Alice':32, 'Bob':48}

Keyword arguments-requires keys are valid python identifiers.
A mapping, such as another dict.

>>> Phonetic = dict(a='alfa', b='bravo')
>>> phonetic

{'a':'alfa', 'b':'bravo'}

d.copy() for copying dictionaries.

>>> e = d.copy()
>>> e

{'Alice':32, 'Bob':48}

Or simply dict(d) constructor

>>> f = dict(e)
>>> f

{'Alice':32, 'Bob':48}

Extend a dictionary with update()

>>> g = dict('c':'charle', 'f':'fort')
>>> f.update(g)
>>> f

{'a':'alfa', 'b':'bravo', 'c':'charle', 'f':'fort'}

Update replaces values corresponding to duplicate keys.
Iteration is over keys.
Get corresponding value with d[key] lookup.

>>> for key in f:
print("{key} => {value}".format(key= key, value=f[key]))

Use values() for an iterable view onto the series of values
No efficient way to get the key corresponding to a value.

>>> for value in f.values():
print(value)

Keys() method gives iterable view onto keys – not often needed.

>>> for key in f.keys():
print(key)

Use items for an iterable view onto the series of key-value tuples.
Use with tuple unpacking.

>>> for key, value in f.items():
print("{key} => {value}".format(key=key, value=value))

The in and not in operators work on the keys.

>>> 'a' in f
>>> 'h' in f

Use del keyword to remove by key. del d[key]

>>> del f['a']

Keys must be immutable.
Values may be mutable.
The dictionary itself is mutable.

Python Part2: Conditional statements, While loops, Strings and Range

Conditional statements


If expr:

     print("expr is true")

If expr:
     print("It's true")

else:
     print("its’s false")

Python provides the elif keyword to eliminate the need for nested if … else structure.


h = 5

if h > 5:
      print("greater than 5")

elif h < 2:
      print("less than 2")

else:
      print("between 2 and 5.")

While loops

# While loop
While expr:
print("loop while expr is true")

# Do-while in python
While True:
If expr:

          Break

Strings

Immutable sequence of Unicode codepoints.

Immutable mean when you construct a string you can't modify its content.

Strings with Newlines

Multiline strings

""" this is
Multiline string"""

Escape sequences

This is \nmultiline \nstring.

len(s) gives number of codepoints(characters).
The + operator can be used for string concatenation.

>>> "This" + "is" + "a" + "string"
Thisisastring

Strings are immutable, so the += operator re-binds the reference to a new object.
Use sparingly, concatenation with + or += can cause performance degradation.
Call the join() method on the separator string.

>>> Numbers = ';'.join([1, 2, 3, 4])
'1;2;3;4'

Use the split() to divide a string into a list.

>>> Numbers.split(';')
[1, 2, 3, 4]

Without an argument, split() divides on whitespace.
Join()-ing on an empty separator is an important and fast way of concatenating a collection of strings.
The partition() method divides a string into three around a separator: prefix, separator, suffix.
Tuple unpacking is useful to destructure the result.

Range

Arithmetic progression of integers.
Stop value is one-past-the-end.

>>> range(5)
range(0, 5)

Ranges are 'half open'-start is include but stop is not.

>>> for I in range(5)
print(I)
0
1
2
3
4

Stop value of a range used as start value of consecutive range.

>>> list(range(5, 10))
[5, 6, 7, 8, 9]
>>> list(range(10, 15))
[10, 11, 12, 13, 14]

Optional third step value.

>>> List(range(0, 10, 2))
[0, 2, 4, 6, 8]

Range(stop) range(10)
Range(start:stop) range(0, 10)
Range(start:stop:step) range(0, 10, 2)

Python Part1: Introduction, Install on windows, Scalar Types and Relational Operators

Introduction to python

Open source programming language developed in late 1980’s.
Strongly typed language in the sense that every object in the language has a definite type.
At the same time, it is dynamically typed means no type checking prior to running it.
General purpose language.
There are some areas where it's less suitable than others, for example, in extremely time sensitive or memory constrained environments.

Install python on windows

https://www.python.org/downloads/

Scalar types

Int
Float
None
bool

Relational operators

== value equality
!= value inequality
< less-than
> greater-than
<= less-than or equal to
>= greater-than or equal to