The python script name and additional arguments thereafter are turned into a list of string and assigned into argv variable in sys module.
- When no script and no argument:
sys.argv[0]is an empty string - When script name is given as -:
sys.argv[0]is set to "-" - When
-cor-mcommand is used:sys.argv[0]is set to-cor-mrespectively
python -c 'print(hello world)'
Here -c stands for a python command
Anything written after -c must be a valid python command
python -m pdb python-101.py
-m stands for module
pdb stands for python debugger
python-101.py is a file containing the python code
python -i python-101.py
-i means, python will run the program, python-101.py and dump you into
interactive interpreter
It helps in monitoring the variables at the end of the program.
To enter directly into python interpreter just type:
python
Another way using python interpreter: Create a file named foo.py
The content of the file foo.py:
#!/usr/bin/env python
print("This is python-101")There are two ways to run this file:
python foo.py- Make it executable
chmod +x foo.pyand run the executable./foo.py
These operator works similar to other languages.
>>> 2 + 2
4
>>> 50 - 5*6
20
>>> (50 - 5*6) / 4
5.0
>>> 8 / 5 # division always returns a floating point number
1.6/always returns a float.//can be used to discard the fractional part%can be used to calculate the remainder**is used to calculate powers=is used to assign a value to a variable
>>> 12 ** 2
144
>>> 2 ** 3
8 >>> a = 12
>>> b = 14
>>> a + b
26
>>> c = a + b
>>> c
26In interactive mode the last printed expression is assigned to a variable named _
- Strings can be enclosed in single quotes (
'...')or double quotes("..."). \Can be used for escaped characters- We can use a combination of single quotes and double quotes to avoid use of
\before escaped character
>>> 'This is BangPypers'
'This is BangPypers'
>>> 'Rama don\'t like python'
'Rama don't like python'
>>> "Hari don't know why"
"Hari don't know why"print()function omits the enclosing quotes.
>>> print("Hari loves python")
Hari loves python- If you want to print the string as it is without interpreting prefaced
\as a special character then use raw strings
>>> print('home\abc')
homebc
>>> print(r'home\abc')
home\abc- If string is too long use triple quotes,
("""...""" or '''...''')
>>> print(""" Python is very easy
... I am learning python
... It is a beautiful language """)
Python is very easy
I am learning python
It is a beautiful language
>>> - Strings can be concatenated using
+and repeated with*
>>> "p" + "y" * 5 + "thon"
'pyyyyython'- Two or more string literals, next to each other are automatically concatenated.
- To concatenate a string literal and a string variable we must use
+
>>> "hello" "world"
'helloworld'
>>> var = "abc"
>>> var + "def"
'abcdef'
>>> var "def" # This will give syntax error
File "<stdin>", line 1
var "def"
^
SyntaxError: invalid syntax- First character of a string has index 0
- Character is a string of size 1
- If index is a negative number, it starts counting form the right
- Negative indices start form -1
>>> word = "This is Python Bangpypers"
>>> word[0] # Indexing starts from 0
'T'
>>> word[1]
'h'
>>> word[5]
'i'
>>> word[12]
'o'
>>> word[-1] # -ve indexing starts form -1 and it points to last character
's'
>>> word[-2]
'r'
>>> word[-5]
'y'
>>> - To obtain substring we use word slicing
- In
word[2:10], character at index2is included and at index10is excluded - Slices have default indices. An omitted first index defaults to
0and omitted 2nd index defaults to size of the string
>>> word[15:-1] # -ve index also works in slicing
'Bangpyper'
>>> word[15:] # last index defaults to length of the string
'Bangpypers'
>>> word[1:]
'his is Python Bangpypers'
>>> word[:] # First index defaults to 0
'This is Python Bangpypers'
>>> word[0:5]
'This '- Python strings are immutable
>>> word[5] = 'a'
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'str' object does not support item assignmentcapitalize(): Return a copy of the string with its first character capitalized and the rest lowercasedcenter(width[, fillchar]): Return centered in a string of length widthcount(sub[, start[, end]]): Return the number of non-overlapping occurrences of substring sub in the range [start, end]find(sub[, start[, end]]): Return the lowest index in the string where substring sub is foundformat(): Returns a copy of the string where each replacement field is replaced with the string value of the corresponding argument.
>>> "The sum of 1 + 2 is {0}".format(1+2)
'The sum of 1 + 2 is 3'isalnum(): Returns true if the string is alphanumericisalpha(),isdecimal(),isdigit(), orisnumeric()join(): Return a string which is the concatenation of the strings, passed as argument in join.
#!/usr/bin/python
s = "-";
seq = ("a", "b", "c"); # This is sequence of strings.
print s.join( seq )split: The method split() returns a list of all the words in the string, using str as the separator optionally limiting the number of splits to num.
#!/usr/bin/python
str = "Line1-abcdef \nLine2-abc \nLine4-abcd";
print str.split( )
print str.split(' ', 1 )
output:
['Line1-abcdef', 'Line2-abc', 'Line4-abcd']
['Line1-abcdef', '\nLine2-abc \nLine4-abcd']- There are many other predefined methods for strings, see here
See the examples below
>>> print("My name is %s, I am %d yrs old" %("rajiv", 15)) # As tuples
My name is rajiv, I am 15 yrs old
>>> print('%(language)s has %(number)03d quote types.' % # As dictionary
... {'language': "Python", "number": 2})
Python has 002 quote types.len(str): This will return the length of the string,str"abc" in str: Will returntrueif "abc" is substring ofstr
str is a string variable
- Comma separated values between square brackets
- Lists can also be indexed and sliced
- List also support concatenation by
+ - Lists are mutable
- To add items at the end of the list use
append()
>>> list = [1, 2, 3, 4, 5, 6]
>>> list
[1, 2, 3, 4, 5, 6]
>>> list[0] # indexing returns the item
1
>>> list[4] # indexing returns the item
5
>>> list[-1] # indexing returns the item
6
>>> list[2:] # Slicing returns a new list
[3, 4, 5, 6]
>>> list + [12, 13, 14] # list support concatenation
[1, 2, 3, 4, 5, 6, 12, 13, 14]
>>> list[4] = 1000 # lists are mutable
>>> list
[1, 2, 3, 4, 1000, 6]
>>> list.append("Bangalore") # Will append bangalore at the end of the list
>>> list
[1, 2, 3, 4, 1000, 6, 'Bangalore']
>>> list[2:4] = ["Python"] # Slices can also be initialized
>>> list
[1, 2, 'Python', 1000, 6, 'Bangalore']
>>> len(list) # length of list can be found using len
6
>>> ### Nesting of list
...
>>>
>>> a = [1, 2, 3]
>>> b = ["m", "n", "o"]
>>> nlist = [a, b]
>>> nlist
[[1, 2, 3], ['m', 'n', 'o']]
>>> nlist[0]
[1, 2, 3]
>>> nlist[1]
['m', 'n', 'o']
>>> nlist[1][0]
'm'
>>> nlist[0][2]
3
- list.append(x): Add an item,
xto the end of thelist - list.extend(L): Append the list,
Ltolist - list.insert(i, x): Insert item,
xat index,iin the givenlist - list.remove(x): Remove item,
xfrom thelist - list.pop(i): remove an item at ith position from the list and return that value. If no index given it returns the last item.
- list.clear(): removes all items form the list
- list.index(x): index of item,
xin the list - list.count(x): returns the number of times item,
xappears in the list - list.reverse(): reverse the elements in the list in place
- list.copy(): returns a copy of the list
- list.sort(reverse=false): sort the list in ascending order if
reverseis false and vice versa
>>> a = [-1, 1, 66.25, 333, 333, 1234.5]
>>> del a[0] # delete an item at ith index
>>> a
[1, 66.25, 333, 333, 1234.5]
>>> del a[2:4] # delete sublist
>>> a
[1, 66.25, 1234.5]
>>> del a[:] # delete whole list
>>> a
[]
>>> del a # delete the entire variable- All tuples are enclosed in paranthesis, to support nested tuples
- But input may or may not be surrounded by parenthesis
- Tuples are immutable
- But they can contain mutable objects, i.e. lists
- Empty tuples are created by empty pair of parenthesis
- Singleton tuples are created by having tuple value followed by a comma.
t = 12345, 54321, 'hello!'is calledtuple packing- the reverse is
tuple unpackingi.e.x, y, z = t.
>>> t = 12345, 54321, 'hello!'
>>> t[0]
12345
>>> t
(12345, 54321, 'hello!')
>>> # Tuples may be nested:
... u = t, (1, 2, 3, 4, 5)
>>> u
((12345, 54321, 'hello!'), (1, 2, 3, 4, 5))
>>> # Tuples are immutable:
... t[0] = 88888
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'tuple' object does not support item assignment
>>> # but they can contain mutable objects:
... v = ([1, 2, 3], [3, 2, 1])
>>> v
([1, 2, 3], [3, 2, 1])>>> empty = ()
>>> singleton = 'hello',
>>> len(empty)
0
>>> len(singleton)
1
>>> singleton
('hello',)Please the examples below
>>> basket = {'apple', 'orange', 'apple', 'pear', 'orange', 'banana'}
>>> print(basket) # show that duplicates have been removed
{'orange', 'banana', 'pear', 'apple'}
>>> 'orange' in basket # fast membership testing
True
>>> 'crabgrass' in basket
False
>>> # Demonstrate set operations on unique letters from two words
...
>>> a = set('abracadabra')
>>> b = set('alacazam')
>>> a # unique letters in a
{'a', 'r', 'b', 'c', 'd'}
>>> a - b # letters in a but not in b
{'r', 'd', 'b'}
>>> a | b # letters in either a or b
{'a', 'c', 'r', 'd', 'b', 'm', 'z', 'l'}
>>> a & b # letters in both a and b
{'a', 'c'}
>>> a ^ b # letters in a or b but not both
{'r', 'd', 'b', 'm', 'z', 'l'}- Called associative arrays in other languages
- They are indexed by user defined keys
- Keys are immutable
- Keys can only be strings and numbers
- Tuples can also be used as keys if they contain only strings and numbers
- Any mutable object cannot be used as a key, e.g lists, slices etc
- Dictionaries can be defined as unorderd set of "key, value" pairs
- it is possible to delete a "key, value" pair using del key
list(d.keys())returns the list of keys of dictionarydsorted(d.keys())to get the keys in sorted order- to check if
keyis present in dictionary we can useinkeyword
Some examples
>>> score = {'harry': 75, 'snape': 97}
>>> score['ron'] = 50
>>> score
{'snape': 97, 'ron': 50, 'harry': 75}
>>> score['harry']
75
>>> del score['snape']
>>> score['voldemort'] = 60
>>> score
{'ron': 50, 'voldemort': 60, 'harry': 75}
>>> list(score.keys())
['voldemort', 'ron', 'harry']
>>> sorted(score.keys())
['ron', 'voldemort', 'harry']
>>> 'ron' in score
True
>>> 'harry' not in score
False- used to build dictionary form a sequence of "key, value" pairs
- It can also be done by specifying pairs using keyword arguments
>>> dict([('sape', 4139), ('guido', 4127), ('jack', 4098)])
{'sape': 4139, 'jack': 4098, 'guido': 4127}
>>> dict(sape=4139, guido=4127, jack=4098)
{'sape': 4139, 'jack': 4098, 'guido': 4127}Lets see this with the help of an example
>>> # first n Fibbonaci numbers
>>> n = 20 # single assignment
>>> a, b = 0, 1 # multi assignment
>>> while n > 0:
... print(a)
... a, b, n = b, a+b, n-1 # multi assignment
...
0
1
1
2
3
5
8
13
21
#deleting the rest.- we can assign variables in the same line as shown in above example
printis printing each output in different lines
- any non-zero integer value evaluates to
trueand zero isfalse - condition can also be a string or list value (anything with non-zero length is
true) - Empty list, string or any sequence is false
- standard comparison operators are:
<, >, ==, <=, >=, and !=
- you can print any number of values using print statement followed by end of line
>>> a, b = 10, 12
>>> print("value of a is: ", a, "value of b is: ", b)
value of a is: 10 value of b is: 12- To avoid end of line use the keyword argument
end.
>>> a, b = 0, 1
>>> while b < 1000:
... print(a, end=',')
... a, b = b, a+b
...
0, 1,1,2,3,5,8,13,21,34,55,89,144,233,377,610,987,See the example below
>>> # Program to find if number is multiple of 2 or 3
...
>>> x = int(input("Please enter an integer number: "))
Please enter an integer number: 56
>>> if x%2 == 0:
... print("x is a multiple of 2")
... elif x%3 == 0:
... print("x is a multiple of 3")
... else:
... print("x is not a multiple of 2 or 3")
...
x is a multiple of 2- There can be zero or more
elifblock andelseblock is optional
- Input is taken as a raw string which is then typecasted into its respective type.
See the example below to understand about for statement in python
>>> ## To capitalize each statement in a string
... stringList = ["ravi got his pocket money.",
"he decided to buy an ice cream.",
"he ran to the ice ream shop, but in hurry he forgot the money",
"ravi came back home and kept the money in his piggy bank"]
>>> for str in stringList:
... print(str.capitalize())
...
Ravi got his pocket money.
He decided to buy an ice cream.
He ran to the ice cream shop, but in hurry he forgot the money
Ravi came back home and kept the money in his piggy bank- If the sequence itself needs to be modified while looping then its recommended to create a copy of same sequence using slice.
>>> for w in words[:]: # Loop over a slice copy of the entire list.
... if len(w) > 6:
... words.insert(0, w)
...
>>> words
['defenestrate', 'cat', 'window', 'defenestrate']- Use
range()function range()can be used in three waysrange(n): will contain numbers form0throughn-1range(x, y): will start fromxand end aty - 1range(x, y, z): will start atxand continue asx + z,x + 2zuntilx + kzis less thany
range(5, 10)
5 through 9
range(0, 10, 3)
0, 3, 6, 9
range(-10, -100, -30)
-10, -40, -70To iterate over indices of a sequence
>>> a = ['Mary', 'had', 'a', 'little', 'lamb']
>>> for i in range(len(a)):
... print(i, a[i])
...
0 Mary
1 had
2 a
3 little
4 lambTo iterate over dictionary
>>> knights = {'gallahad': 'the pure', 'robin': 'the brave'}
>>> for k, v in knights.items():
... print(k, v)
...
gallahad the pure
robin the brave>>> for i, v in enumerate(['tic', 'tac', 'toe']):
... print(i, v)
...
0 tic
1 tac
2 toe
>>> for i in reversed(range(1, 10, 2)):
... print(i)
...
9
7
5
3
1- To convert
rangeinto a list uselist(range(5))
break: breaks out of the loopcontinue: continues with the next iteration of the loopelse: It is executed when loop terminates through exhaution of the list (in case offor) or when loops condition becomes false (in case ofwhile)
>>> for n in range(2, 10):
... for x in range(2, n):
... if n % x == 0:
... print(n, 'equals', x, '*', n//x)
... break
... else:
... # loop fell through without finding a factor
... print(n, 'is a prime number')
...
2 is a prime number
3 is a prime number
4 equals 2 * 2
5 is a prime number
6 equals 2 * 3
7 is a prime number
8 equals 2 * 4
9 equals 3 * 3- It does nothing
>>> if True:
... pass
>>>Keyword def is used to create new function.
>>> def sum_n(n):
... """ Prints sum of n natural numbers. """
... print(n * (n + 1) / 2)
...def is followed by function name sum_n, followed by arguements in parens n.
Statements followed by definition form body of function and they should be indented properly.
It can also have doc string to summarize what a function does. This is optional.
You can check whether an object is a function.
>>> type(sum_n)
<class 'function'>Calling a function with approriate arguements executes the function.
>>> sum_n()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: sum_n() missing 1 required positional argument: 'n'
>>> sum_n(5)
15.0Functions are objects. They can be passed around just like variables.
>>> add_n = sum_n
>>> add_n(5)
15.0Every function returns some value.
If user doesn't return any value, None will be returned.
>>> x = sum_n(5)
15.0
>>> print(x)
NoneValues can be returned from function using return statement.
We can modify previous function to return value instead of printing it.
>>> def sum_n(n):
... """ Returns sum of n natural numbers. """
... return (n * (n + 1) / 2)
...
>>> x = sum_n(5)
>>> print(x)
15.0Arguements can be either positional or keyword.
>>> def power(base, exponent):
... return pow(base, exponent)
...
>>> power(2, 3)
8
>>> power(base=2, exponent=3)
8Keyword arguements can be passed in any order.
>>> power(exponent=3, base=2)
8
>>> power(base=2)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: power() missing 1 required positional argument: 'exponent'We can assign default values to arguements to make them optional.
>>> def power(base, exponent=1):
... return pow(base, exponent)
...
>>> power(2)
2
>>> power(2, 3)
8
>>> power(exponent=3, base=2)
8Python has some builtin functions which are always available.
>>> help()
>>> print('python')
python
>>> score = [45, 67, 89, -12]
>>> sum(score)
189
>>> len(score)
4
>>> max(score)
89
>>> min(score)
-12
>>> range(5)
range(0, 5)
>>> list(range(5))
[0, 1, 2, 3, 4]
>>> pow(3, 4)
81Full list of functions can be found here
Lets try to understand Lambda.
Small anonymous functions can be created with the lambda keyword
def sumof(x,y):
return x+yAbove function can also be represented using lambda
lambda x,y: x+yLambda functions can be used wherever function objects are required. Said that, let us try some inbuilt
functions like map, filter
As help help of map
map(func, *iterables) --> map object
|
| Make an iterator that computes the function using arguments from
| each of the iterables. Stops when the shortest iterable is exhausted.we either can define a function first and then pass it as first param, or use lambda instead.
map(sumof, [1,2,3], [4,5,6])or
map(lambda x,y: x+y, [1,2,3], [4,5,6])Files can read/write using open function.
You should pass filename(string) as argument.
You can also pass mode('r', 'w'), which is optional.
>>> f = open('data.txt')
>>> f
<open file 'data.txt', mode 'r' at 0x7fd3513d54b0>
>>> f = open('data.txt')
>>> for line in f:
... print(line)
...
If you are reading this using python,
You are on your way to become awesome python programmer.
Now that you have read the file,
try writing some text into a file.
>>>
Lets create and write something into a file.
>>> f = open('story.txt')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IOError: [Errno 2] No such file or directory: 'story.txt'
>>> f = open('story.txt', 'w')
>>> f.write('Once there lived a python programmer.')
>>> f.write('foo bar baz.')
>>> f.write('more foo')
>>> f.close()
A class is a specification (think of it as a blueprint or pattern and a set of instructions) of how to provide some service. Engineers and construction and factory workers use blueprints to build cars, buildings, bridges, virtually anything. Tailors, seamsters, printers use patterns to make the clothes we wear, books we read. Chefs follow recipes to put together meals.
>>> class Maths:
def sumof(self, x, y):
return x+y
def mulof(self, x, y):
return x*y
>>> obj = Maths()
>>> obj.sumof(3,4)
7
>>> obj.mulof(3,4)
12Common mistakes while coding during initial days
>>> class Maths:
def sumof(x, y):
return x+y
def mulof(x, y):
return x*y
>>> obj = Maths()
>>> obj.sumof(3,4)
Traceback (most recent call last):
File "<pyshell#49>", line 1, in <module>
obj.sumof(3,4)
TypeError: sumof() takes 2 positional arguments but 3 were given
Below example from taken from http://anandology.com/python-practice-book/object_oriented_programming.html#classes-and-objects
class BankAccount:
def __init__(self):
self.balance = 0
def withdraw(self, amount):
self.balance -= amount
return self.balance
def deposit(self, amount):
self.balance += amount
return self.balance
>>> a = BankAccount()
>>> b = BankAccount()
>>> a.deposit(100)
100
>>> b.deposit(50)
50
>>> b.withdraw(10)
40
>>> a.withdraw(10)
90
class BankAccount:
def __init__(self, initial_balance=0):
self.balance = initial_balance
def withdraw(self, amount):
self.balance -= amount
return self.balance
def deposit(self, amount):
self.balance += amount
return self.balance
>>> obj = BankAccount(200)
>>> obj.balance
200
>>> obj.withdraw(50)
150
>>> obj.deposit(150)
300Use docstring while writing code, that can be used to create documentation or while doing help to the object
>>> class BankAccount:
"""This class can be used to get the bank account details
and also to do transaction.
"""
def __init__(self, initial_balance=0):
"""Account holder can open their account with certail
Initial balance, otherwise initial balace will be 0
"""
self.balance = initial_balance
def withdraw(self, amount):
"""This function can be used to withdraw amount from
your account.
"""
self.balance -= amount
return self.balance
def deposit(self, amount):
"""This function can be used to deposit amount into
your account.
"""
self.balance += amount
return self.balance
>>> help(BankAccount)
Help on class BankAccount in module __main__:
class BankAccount(builtins.object)
| This class can be used to get the bank account details
| and also to do transaction.
|
| Methods defined here:
|
| __init__(self, initial_balance=0)
| Account holder can open their account with certail
| Initial balance, otherwise initial balace will be 0
|
| deposit(self, amount)
| This function can be used to deposit amount into
| your account.
|
| withdraw(self, amount)
| This function can be used to withdraw amount from
| your account.A language feature would not be worthy of the name “class” without supporting inheritance. The syntax for a derived class definition looks like this:
class DerivedClassName(BaseClassName):
<statement-1>
.
.
.
<statement-N>Example
>> class Alto:
def price(self):
return '3L INR'
def power(self):
return '800cc'
def maker(self):
return 'Maruti'
>>> obj = Alto()
>>> obj.maker()
'Maruti'
>>> obj.power()
'800cc'
>>>
>>>
>>> class Swift(Alto):
def new_features(self):
return 'airbag, bluetooth'
>>> sw_obj = Swift()
>>> sw_obj.maker()
'Maruti'
>>> sw_obj.power()
'800cc'
>>> sw_obj.new_features()
'airbag, bluetooth'
>>>
>>> class Swift_newgen(Swift):
def price(self):
return '5L INR'
def power(self):
return '1200cc'
>>> ng_obj = Swift_newgen()
>>> ng_obj.price()
'5L INR'
>>> ng_obj.power()
'1200cc'
>>> ng_obj.maker()
'Maruti'
>>> Note: Some of the examples in this tutorial are taken form official documentation/tutorial of python3. [See here] https://docs.python.org/3/tutorial/