1. Learn Python – for beginners
Part-I
Raj Kumar Rampelli

2. Outline
• Introduction
• Variables
• Strings
• Command line arguments
• if statement
• while loop
• Functions
• File Handling
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3. Python Introduction
• Python is a high level programming language like a C/C++/Java etc..
• Python is processed at runtime by Interpreter
– No need to compile the python program.
• Python source file has an extension of .py
• Python versions: 1.x, 2.x and 3.x
– Both 2.x (Python) and 3.x (Python3) are currently used
• Python console:
– Allow us to run one line of python code, executes it and display the
output on the console, repeat it (Read-Eval-Print-Loop)
– quit() or exit() to close the console
• Python applications including web, scripting, computing and
artificial intelligence etc.
• Python instruction/code doesn’t end with semicolon ; and it
doesn’t use any special symbol for this.
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4. Write Python code – Variables usage
• Variables don’t have any specific data type here like int/float/char etc.
– A variable can be assigned with different type of values in the same program
• Adding comments in program
– # symbol is used to add a single line comment in the program (symbol // in C)
– “”” “”” used for multiple line comment here (/* */ in C)
• Python is case sensitive, i.e. Last and last are two different variables.
• Variable name should have only letters, numbers, underscore and
theyu can’t start with numbers.
• NameError:
– Occur when program tries to access a variable which is not defined in the program.
• del statement used to delete a variable
– Syntax: del variable_name
#Save below code in sample.py and run
A = 100
print(A)
A = “Python”
print(A)
Output:
100
‘Python’
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5. Strings
• String is created by entering text between single quotes or double quotes.
“Python” is a string and ‘Python’ is a string.
String basic operations
Concatenation
str() is a special function that
converts input to string.
"spam"+"eggs" -> output: spameggs
"spam"+","+"eggs" -> output: spam,eggs
"2"+"2" output: 22
1 + "2" -> output: TypeError
str(1) + “2” -> output: 12
Multiplication "spam"*3 -> output: spamspamspam
4 * "3" -> output: 3333
"s" * "r" -> output: TypeError
"s" * 7.0 -> output: TypeError
Replace "hello me".replace("me", "world") -> output: hello world
Startswith "This is car".startswith("This") -> output: True
Endswith "This is car".endswith("This") -> output: False
Upper() "I am a boy".upper() -> output: I AM A BOY
Lower() ”I am a Boy”.lower() -> output: i am a boy
Split() "spam, eggs, ham".split(",") -> output: ['spam', ' eggs', ' ham']6/16/2016 Rajkumar Rampelli - Learn Python 5

6. Command line arguments
• Python sys module provides a way to access command line arguments
– sys.argv is a list containing all arguments passed via command line arguments
and sys.argv[0] contains the program name
#!/usr/bin/python
import sys
“””len() is a special function that returns the number of characters in a
string or number of strings in a list.”””
#To avoid concatenation error, converted length into string using str().
print('Number of arguments:‘ + str(len(sys.argv)) + 'arguments.‘)
print('Argument List:‘ + str(sys.argv))
Run: sample.py arg1 arg2 arg3
Output:
Number of arguments: 4 arguments
Argument List: [‘sample.py', 'arg1', 'arg2', 'arg3']
Multiline
comment
(“”” text
“””)
Single line
comment
(#)
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7. if statement
• Python uses indentation (white space at the
beginning of a line) to delimit the block of
code. Syntax:
• elif - short form of else if
– Or use standard way
if expression:
statement
else:
if expression:
statement
if condition:
line1
line2
elif condition2:
line4
else:
line5
Print(“hello”)
White
space
or tab
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8. while loop
• Use it when run a code for certain number of times. Syntax:
• infinite loop –
while 1==1:
print("in the loop")
• break - To end a while loop prematurely,
then break statement can be used.
i = 1
while 1==1:
if i > 5:
break
print("in the loop")
i = i + 1
• Continue - Unlike break, continue jumps back to the top of the
loop, rather than stopping it.
• break and continue usage is same across other languages (ex: C)
while condition:
statement1
statement2
It will print “In the
loop” for 4 times.
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9. else with loops
• Using else with for and while loops, the code
within it is called if the loop finished normally
(when a break statement doesn’t cause an exit
from the loop).
i = 50
while i < 100:
if i % 3 == 4:
print(“breaking”)
break
i = i + 1
else:
print(“Unbroken”)
Output:
Unbroken
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10. else with try/except
• The else statement can also be used with
try/except statements. In this case, the code
within it is only executed if no error occurs in
the try statement.
try:
print(1)
except ZeroDivisionError:
print(2)
else:
print(3)
Output:
1
3
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11. User defined functions
• create a function by using the def statement
and must be defined before they are called,
else you would see NameError.
• Functions can be assigned and reassigned to
variables and later referenced by those values
• The code in the function must be indented.
def my_func():
print("I am in function")
my_func()
func2 = my_func()
print(“before func2”)
func2()
Output:
I am in function
before func2
I am in function
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12. Functions with arguments and return
value
def max(x, y):
if x >= y:
return x
else:
return y
z = max(8, 5)
print(z)
Output:
8
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13. File handling
Open file:
myfile = open("filename.txt", mode);
The argument of the open() function is the path to
the file. You can specify the mode used to open a file
by 2nd argument.
r : Open file in read mode, which is the default.
w : Write mode, for re-writing the contents of the file
a : Append mode, for adding new content to the end
of the file
b : Open file in a binary mode, used for non-text files.
Writing Files - write()
write() - writes a string to the file.
"w" mode will create a file if not
exist. If exist, the contents of the
file will be deleted.
write() returns the number of
bytes written to the file, if
successful.
file = open("new.txt", "w")
file.write("Writting to the file")
file.close()  closes file.
Reading file:
1. read() : reads entire file
2. readline() : return a list in which each
element is a line in the file.
Example:
cont = myfile.read()
print(cont) -> print all the contents of the file.
print(“Before readline()”)
cont2 = myfile.readline()
print(cont2)
Input file contains:
Line1
line2
Output:
Line1
line2
Before readline()
[“line1”, “line2”]
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14. C language Vs Python
C language Python language
Special operators (++ and --) works
I.e. a++; --a;
It doesn't support ++ and --. Throws Syntax error.
Each statement in C ends with semicolon ; No use of ; here.
Curly braces are used to delimit blocks of code
If (condition)
{
Statement1;
Statement2;
}
It uses white spaces for this purpose
If condition:
Statement1
Statement2
Compiling the program before execution is mandatory Python program directly executed by using interpreter. No
compiler here.
Boolean operators are && and || and ! Boolean operators are and, or and not
Uses // for one line comment
Uses /* */ for multiple line comment
Uses # for one line comment
Uses """ """ for multi line comments (Docstrings).
Uses #include to import standard library functions
#include<stdio.h>
Uses import keyword to include standard library functions
Import math
Void assert(int expression)
Expression -This can be a variable or any C expression. If
expression evaluates to TRUE, assert() does nothing.
If expression evaluates to FALSE, assert() displays an error
message on stderr(standard error stream to display error
messages and diagnostics) and aborts program execution.
assert expression
Example
assert 1 + 1 == 3
NULL – represents absence of value None - represents absence of value
Don't have automatic memory management. Automatic Garbage collection exist
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15. Next: Part-2 will have followings. Stay Tune..!!
• Data Structures
– Lists
– Sets
– Dictionaries
– Tuples
• Exception Handling
• Python modules
• Regular expressions – tool for string manipulations
• Standard libraries
• Python Programs
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16. References
• Install Learn Python (SoloLearn) from Google
Play :
https://play.google.com/store/apps/details?id
=com.sololearn.python&hl=en
• Learn Python the Harder Way :
http://learnpythonthehardway.org/
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17. Thank you
• Have a look at
• My PPTs:
http://www.slideshare.net/rampalliraj/
• My Blog: http://practicepeople.blogspot.in/
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