PythonBasics Of Python
Variables in Python
A variable is a named container used to store data (value).
In Python, variables are created automatically when a value is assigned to them.
Example:
x = 2 # x is the variable, 2 is the value
print(x + 3) # Output: 5
y = 3
print(x + y) # Output: 5- Variables are mutable, meaning their values can be changed during execution.
x = 9
print(x) # Output: 9
print(x + y) # Output: 12Accessing Undefined Variables
If you try to use a variable that is not defined, Python throws a NameError.
Example:
abc
# Traceback (most recent call last):
# NameError: name 'abc' is not defined. Did you mean: 'abs'?Using the Underscore _
Python automatically stores the result of the last executed expression in the variable _.
Example:
x = 5
y = 3
x + y # Output: 8
_ + y # Output: 11 (since _ stores previous output 8)Strings as Variables
- Strings in Python can be written using single (‘ ’) or double (“ ”) quotes.
- Variables can store string values just like numbers.
Example:
name = "youtube"
print(name) # Output: youtube
print(name + " rocks") # Output: youtube rocksIncorrect concatenation:
print(name ' rocks') # SyntaxErrorString Indexing and Slicing
(a) Positive Indexing
Each character in a string has an index:
y o u t u b e
0 1 2 3 4 5 6(b) Negative Indexing
Python also supports negative indexing:
y o u t u b e
-7 -6 -5 -4 -3 -2 -1Examples:
name = "youtube"
print(name[0]) # 'y'
print(name[6]) # 'e'
print(name[-1]) # 'e'
print(name[0:2]) # 'yo' (characters from index 0 to 1)
print(name[1:4]) # 'out'
print(name[1:]) # 'outube' (from index 1 till end)
print(name[:4]) # 'yout'
print(name[3:10]) # 'tube' (index out of range is ignored)
print(name[-5:-2]) # 'utu'Special Cases:
print(name[:]) # 'youtube' (entire string)
print(name[:0]) # '' (empty string)Strings are Immutable
In Python, strings cannot be modified once created.
Attempting to assign new characters to existing string indices results in an error.
Example:
name = "youtube"
name[0] = "r"
# TypeError: 'str' object does not support item assignmentString Concatenation
Strings can be joined using the + operator.
Example:
name = "youtube"
print("my " + name[3:]) # Output: my tubeBuilt-in Function: len()
The len() function returns the total number of characters in a string.
Example:
myname = "Navin Reddy"
print(len(myname)) # Output: 11Summary
- Variables are containers for values and can be reassigned.
- Using an undefined variable raises a NameError.
_stores the last evaluated result.- Strings can be assigned, concatenated, sliced, and indexed.
- Strings are immutable (cannot be changed after creation).
len()is used to find the length of a string.