Python Basics: Operators and Syntax

Goals of this lecture

• What is syntax?

• Key operators in Python.

  • Math.

  • Logic.

  • Comparisons.

  • Identity.

  • Membership.

• Indentation in Python.

Python Syntax: Overview

The syntax of a programming language is the set of rules about how different symbols can be combined to produce correctly structured statements.

Like other programming languages, Python has particular syntactic rules.

• Failure to follow these rules results in a SyntaxError.

• Although following syntactic rules can sometimes seem annoying, remember that there’s always a reason the language was designed in a certain way.

Syntax goes hand-in-hand with the operators we use in a language, and the rules about how those operators are used.

Operators

An operator is used to perform an operation on variables and values.

We’ve already seen an example of an operator: = is used to assign a variable name to some value.

### Assignment operator
x = 10

But operators can also include basic arithmetic operations, like addition (+) and subtraction (-).

### Addition operator
1 + 1

2

Arithmetic in Python

Python code can be used to perform arithmetic calculations with numeric values, including:

Operation	Symbol
Addition	+
Subtraction	-
Division	/
Multiplication	*
Exponentiation	**
Modulus	%
Floor division	//

### Exponentiation
2 ** 3

8

### Division
2 / 4

0.5

### Modulus
8 % 3

2

Order of operations

If a single line of code has multiple operations, Python executes these operations according to PEMDAS.

• E.g., () first, then **, then *//, then +/-.

• Word of caution: it’s easy to misplace parentheses (())––many experienced programmers can introduce bugs this way.

Check-in

What value would x take on in the following code?

x = (1 + 2) / (18 - 3)

### Your code here

Check-in

What about this code?

x = 1 + 2 / 18 - 3

### Your code here

“Adding” strings

The + operator can also be applied to strings. In this case, it concatenates the strings (i.e., puts them together).

We’ll revisit this soon when we discuss strings in more depth.

### Addition "concatenates" strings
"a" + "pple"

'apple'

Assignment in Python

We’ve already learned about the basic assignment operator: the symbol = can be used to assign a value to a variable name.

There are also a few “syntactic tricks” with this operator, such as:

Operation	Symbol	Example
Add to variable	+=	x += 1
Subtract from	+=	x -= 1

These are equivalent to just writing out something like: x = x + 1.

x = 10
x += 1
x

11

Logical operators

Logical operators can be used to produce a boolean value. They are particularly useful when writing conditional statements, which we’ll discuss soon.

Symbol	Description	Example
and	Returns True if both parts are true	True and True
or	Returns True if at least one part is true	True or False
not	Returns the reverse	not True

True and False

False

True or False

True

True and (not False)

True

Comparison operators

A comparison operator compares one value to another. This includes whether those values are the same, but also whether one is larger or smaller than the other, and so on.

Symbol	Description
==	Equal
!=	Not Equal
>	Greater Than
<	Less Than
>=	Greater Than or Equal To
<=	Less Than or Equal To

## Equal operator
2 == (1 + 1)

True

## Greater than
2 > (1 + 1)

False

Comparing strings

Note that these operators can also be applied to strings.

• The equality operator (==) tests whether the two strings have the same characters.

• The greater/less than operators (> and <) test the relative ordinal value of the strings, i.e., if they were to be sorted.

## Are these strings equal?
"test" == "test"

True

## Is b "larger" than a?
"b" > "a"

True

## Is ab "larger" than aa?
"ab" > "aa"

True

Check-in

Would the following code return True or False?

"bat" > "cat"

### Your code here

Identity operators

An identity operator determines whether two objects are identical or not. There are just two symbols:

Symbol	Description
is	Identical
is not	Not Identical

Note that identity is not exactly the same as equality.

• A test for equality (==) checks whether two values are the same.

• A test for identity (is) checks whether two operands point to the same object in memory.

  • You don’t need to know all the details here––the most important thing is that they’re subtly different.

# Comparing equality vs. identity
a = "This is a fairly long string"
b = "This is a fairly long string"
print(a == b)
print(a is b)

True
False

Identity vs. Equality: The details

Two variables can have the same value (they’re equal), but reference different objects in memory (i.e., they’re not identical). We can access the id of an object using id(x).

x = 1000
y = 1000
print(id(x))
print(id(y))
print(x == y)
print(x is y)

140479810980944
140479810981136
True
False

Behind the scenes, Python creates objects in memory whenever we declare a new variable referencing a value, with some exceptions:

• Simple/short strings.

• Integers between -5 and 256

x = 1
y = 1
print(x is y)
print(x == y)

True
True

Membership operators

A membership operator determines whether a given value or variable is present within a larger sequence.

Symbol	Description
in	Is the variable/value in the sequence?
not	Is the variable/value not in the sequence?

This will become clearer when we discuss different kinds of sequences, such as strings (str) and lists (list). For now, it’s enough to compare/contrast the examples below.

print("a" in "apple")
print("b" in "apple")

True
False

print("a" not in "apple")
print("b" not in "apple")

False
True

Indentation in Python

In Python, indentation matters for how different blocks of code get evaluated.

• Everything within an indented block gets interpreted as happening “within” that block (e.g., within a loop).

• This will make more sense when we discuss conditional logic (if/else) and loops (e.g., a for loop).

• If you indent where it’s not necessary or expected, you’ll get a SyntaxError.

## We shouldn't have indented here
    print("Don't indent here")

  Input In [24]
    print("Don't indent here")
    ^
IndentationError: unexpected indent

## It's appropriate to indent after a conditional statement
x = 2 - 1
if x == 1:
    print("This is an indented block")

This is an indented block

Conclusion

As always, there’s more we could discuss. However, hopefully this was a useful introduction to the notion of an operator in Python, along with some of the syntactic rules around how to use these operators in code.