BASIC SYNTAX

1.RESERVED WORDS:  Keywords are the reserved words in Python. We cannot use a keyword as a variable name, function name or any other identifier. They are used to define the syntax and structure of the Python language.In Python, keywords are case sensitive (upper case letters are different from lower case letters). There are 33 keywords in Python 3.7. This number can vary slightly over the course of time. All the keywords except True, False and None are in lowercase and they must be written as they are. The list of all the keywords is given below.

False	await	else	import	pass
None	break	except	in	raise
True	class	finally	is	return
and	continue	for	lambda	try
as	def	from	nonlocal	while
assert	del	global	not	with
async	elif	if	or	yield

2.PYTHON IDENTIFIERS: An identifier is a name given to entities like class, functions, variables, etc. It helps to differentiate one entity from another.

Rules for writing identifiers :

1. Identifiers can be a combination of letters in lowercase (a to z) or uppercase (A to Z) or digits (0 to 9) or an underscore _. Names like myClass, var_1 and print_this_to_screen, all are valid example.
2. An identifier cannot start with a digit. 1variable is invalid, but variable1 is a valid name.
3. Keywords cannot be used as identifiers. Eg: global = 1 is invalid
4. Output

5. File "<interactive input>", line 1
       global = 1
              ^
   SyntaxError: invalid syntax

6. We cannot use special symbols like !, @, #, $, % etc. in our identifier.

   a@ = 0

1. Output

1.   File "<interactive input>", line 1
       a@ = 0
        ^
   SyntaxError: invalid syntax

2. An identifier can be of any length.

3.LINES AND INDENTATIONS: Python provides no braces to indicate blocks of code for class and function definitions or flow control. Blocks of code are denoted by line indentation, which is rigidly enforced.

The number of spaces in the indentation is variable, but all statements within the block must be indented the same amount. For example −

if True:
   print "True"
else:
   print "False"

However, the following block generates an error −

if True:
print "Answer"
print "True"
else:
print "Answer"
print "False"

Thus, in Python all the continuous lines indented with same number of spaces would form a block.

4.INPUT OUTPUT FUNCTIONS:

Python provides numerous built-in functions that are readily available to us at the Python prompt.

Some of the functions like input() and print() are widely used for standard input and output operations respectively. Let us see the output section first.

Python Output Using print() function

We use the print() function to output data to the standard output device (screen). An example of its use is given below.

print('This sentence is output to the screen')

Output

This sentence is output to the screen

Another example is given below:

a = 5

print('The value of a is', a)

Output

The value of a is 5

In the second print() statement, we can notice that space was added between the string and the value of variable a. This is by default, but we can change it.

The actual syntax of the print() function is:

print(*objects, sep=' ', end='\n', file=sys.stdout, flush=False)

Here, objects is the value(s) to be printed.

The sep separator is used between the values. It defaults into a space character.

After all values are printed, end is printed. It defaults into a new line.

The file is the object where the values are printed and its default value is sys.stdout (screen). Here is an example to illustrate this.

print(1, 2, 3, 4)

print(1, 2, 3, 4, sep='*')

print(1, 2, 3, 4, sep='#', end='&')

Output

1 2 3 4

1*2*3*4

1#2#3#4&

---

Output formatting

Sometimes we would like to format our output to make it look attractive. This can be done by using the str.format() method. This method is visible to any string object.

>>> x = 5; y = 10

>>> print('The value of x is {} and y is {}'.format(x,y))

The value of x is 5 and y is 10

Here, the curly braces {} are used as placeholders. We can specify the order in which they are printed by using numbers (tuple index).

print('I love {0} and {1}'.format('bread','butter'))

print('I love {1} and {0}'.format('bread','butter'))

Output

I love bread and butter

I love butter and bread

We can even use keyword arguments to format the string.

>>> print('Hello {name}, {greeting}'.format(greeting = 'Goodmorning', name = 'John'))

Hello John, Goodmorning

We can also format strings like the old sprintf() style used in C programming language. We use the % operator to accomplish this.

>>> x = 12.3456789

>>> print('The value of x is %3.2f' %x)

The value of x is 12.35

>>> print('The value of x is %3.4f' %x)

The value of x is 12.3457

---

Python Input

Up until now, our programs were static. The value of variables was defined or hard coded into the source code.

To allow flexibility, we might want to take the input from the user. In Python, we have the input() function to allow this. The syntax for input() is:

input([prompt])

where prompt is the string we wish to display on the screen. It is optional.

>>> num = input('Enter a number: ')

Enter a number: 10

>>> num

'10'

Here, we can see that the entered value 10 is a string, not a number. To convert this into a number we can use int() or float() functions.

>>> int('10')

10

>>> float('10')

10.0

This same operation can be performed using the eval() function. But eval takes it further. It can evaluate even expressions, provided the input is a string

>>> int('2+3')

Traceback (most recent call last):

  File "<string>", line 301, in runcode

  File "<interactive input>", line 1, in <module>

ValueError: invalid literal for int() with base 10: '2+3'

>>> eval('2+3')

5