An expression is something that can be reduced to a value, for example "1+3" is an expression, but "foo = 1+3" is not.

It’s easy to check:

print(foo = 1+3)

If it doesn’t work, it’s a statement, if it does, it’s an expression.

Another statement could be:

class Foo(Bar): pass

as it cannot be reduced to a value.

Though this isn’t related to Python:

An expression evaluates to a value.
A statement does something.

>>> x + 2         # an expression
>>> x = 1         # a statement 
>>> y = x + 1     # a statement
>>> print y       # a statement (in 2.x)
2

Python calls expressions “expression statements”, so the question is perhaps not fully formed.

A statement consists of pretty much anything you can do in Python: calculating a value, assigning a value, deleting a variable, printing a value, returning from a function, raising an exception, etc. The full list is here: http://docs.python.org/reference/simple_stmts.html#

An expression statement is limited to calling functions (e.g.,
math.cos(theta)”), operators ( e.g., “2+3”), etc. to produce a value.

Expression — from the New Oxford American Dictionary:

expression: Mathematics a collection
of symbols that jointly express a
quantity : the expression for the
circumference of a circle is 2πr.

In gross general terms: Expressions produce at least one value.

In Python, expressions are covered extensively in the Python Language Reference In general, expressions in Python are composed of a syntactically legal combination of Atoms, Primaries and Operators.

Python expressions from Wikipedia

Examples of expressions:

Literals and syntactically correct combinations with Operators and built-in functions or the call of a user-written functions:

>>> 23
23
>>> 23l
23L
>>> range(4)
[0, 1, 2, 3] 
>>> 2L*bin(2)
'0b100b10'
>>> def func(a):      # Statement, just part of the example...
...    return a*a     # Statement...
... 
>>> func(3)*4
36    
>>> func(5) is func(a=5)
True

Statement from Wikipedia:

In computer programming a statement
can be thought of as the smallest
standalone element of an imperative
programming language. A program is
formed by a sequence of one or more
statements. A statement will have
internal components (e.g.,
expressions).

Python statements from Wikipedia

In gross general terms: Statements Do Something and are often composed of expressions (or other statements)

The Python Language Reference covers Simple Statements and Compound Statements extensively.

The distinction of “Statements do something” and “expressions produce a value” distinction can become blurry however:

• List Comprehensions are considered “Expressions” but they have looping constructs and therfore also Do Something.
• The if is usually a statement, such as if x<0: x=0 but you can also have a conditional expression like x=0 if x<0 else 1 that are expressions. In other languages, like C, this form is called an operator like this x=x<0?0:1;
• You can write you own Expressions by writing a function. def func(a): return a*a is an expression when used but made up of statements when defined.
• An expression that returns None is a procedure in Python: def proc(): pass Syntactically, you can use proc() as an expression, but that is probably a bug…
• Python is a bit more strict than say C is on the differences between an Expression and Statement. In C, any expression is a legal statement. You can have func(x=2); Is that an Expression or Statement? (Answer: Expression used as a Statement with a side-effect.) The assignment statement of x=2 inside of the function call of func(x=2) in Python sets the named argument a to 2 only in the call to func and is more limited than the C example.

A statement contains a keyword.

An expression does not contain a keyword.

print "hello" is statement, because print is a keyword.

"hello" is an expression, but list compression is against this.

The following is an expression statement, and it is true without list comprehension:

(x*2 for x in range(10))

Statements represent an action or command e.g print statements, assignment statements.

print 'hello', x = 1

Expression is a combination of variables, operations and values that yields a result value.

5 * 5 # yields 25

Lastly, expression statements

print 5*5

Expressions:

• Expressions are formed by combining objects and operators.
• An expression has a value, which has a type.
• Syntax for a simple expression:<object><operator><object>

2.0 + 3 is an expression which evaluates to 5.0 and has a type float associated with it.

Statements

Statements are composed of expression(s). It can span multiple lines.

An expression is something, while a statement does something.

An expression is a statement as well, but it must have a return.

>>> 2 * 2　　　　　　　  　#expression
>>> print(2 * 2)　　　　　#statement

PS:The interpreter always prints out the values of all expressions.

1. An expression is a statement that returns a value. So if it can appear on the right side of an assignment, or as a parameter to a method call, it is an expression.
2. Some code can be both an expression or a statement, depending on the context. The language may have a means to differentiate between the two when they are ambiguous.

STATEMENT:

A Statement is a action or a command that does something. Ex: If-Else,Loops..etc

val a: Int = 5
If(a>5) print("Hey!") else print("Hi!")

EXPRESSION:

A Expression is a combination of values, operators and literals which yields something.

val a: Int = 5 + 5 #yields 10

Statements before could change the state of our Python program: create or update variables, define function, etc.

And expressions just return some value can’t change the global state or local state in a function.

But now we got :=, it’s an alien!

Expressions always evaluate to a value, statements don’t.

e.g.

variable declaration and assignment are statements because they do not return a value

const list = [1,2,3];

Here we have two operands – a variable ‘sum’ on the left and an expression on the right.
The whole thing is a statement, but the bit on the right is an expression as that piece of code returns a value.

const sum = list.reduce((a, b)=> a+ b, 0);  

Function calls, arithmetic and boolean operations are good examples of expressions.

Expressions are often part of a statement.

The distinction between the two is often required to indicate whether we require a pice of code to return a value.

References

Expressions and statements

2.3 Expressions and statements – thinkpython2 by Allen B. Downey

2.10. Statements and Expressions – How to Think like a Computer Scientist by Paul Resnick and Brad Miller

An expression is a combination of values, variables, and operators. A value all by itself is
considered an expression, and so is a variable, so the following are all legal expressions:

>>> 42
42
>>> n
17
>>> n + 25
42

When you type an expression at the prompt, the interpreter evaluates it, which means that
it finds the value of the expression. In this example, n has the value 17 and n + 25 has the
value 42.

A statement is a unit of code that has an effect, like creating a variable or displaying a
value.

>>> n = 17
>>> print(n)

The first line is an assignment statement that gives a value to n. The second line is a print
statement that displays the value of n.
When you type a statement, the interpreter executes it, which means that it does whatever
the statement says. In general, statements don’t have values.

An expression translates to a value.

A statement consumes a value* to produce a result**.

*That includes an empty value, like: print() or pop().

**This result can be any action that changes something; e.g. changes the memory ( x = 1) or changes something on the screen ( print("x") ).

A few notes:

• Since a statement can return a result, it can be part of an expression.
• An expression can be part of another expression.

Expressions and statements are both syntactic constructs in Python’s grammar.

An expression can be evaluated to produce a value.

A statement is a standalone piece of a Python program (which is simply a sequence of 0 or more statements, executed sequentially). Each kind of statement provides a different set of semantics for defining what the program will do.

Statements are often (but not always) constructed from expressions, using the values of the expressions in their own unique way. Usually, there is some keyword or symbol that can be used to recognize each particular kind of statement.

An exhaustive (as of Python 3.11) list of the various kinds of statements.

• An expression statement evaluates an expression and discards the result. (Any expression can be used; there are no other distinguishing features of an expression statement.)
• An assignment statement evaluates an expression and assigns its value to one or more targets. It is identified by the use of = outside of the context of a function call.
• An assert statement evaluates an expression and raises an exception if the expression’s value is false. It is identified by the assert keyword.
• A pass statement does nothing: it’s job is to provide the parser with a statement where one is expected but no other statement is appropriate. It is identified by (and consists solely of) the pass keyword.
• A del statement removes a binding from the current scope. No expression is evaluated; the binding must be a name, an indexed name, or a dotted name. It is identified by the del keyword.
• A return statement returns control (and the value of an optional expression, defaulting to None) to the caller of a function. It is identified by the return keyword.
• A yield statement returns the value of an optional expression (defaulting to None) to the consumer of a generator. It is identified by the yield keyword. (yield is also used in yield expressions; context will make it clear when a statement or expression is meant.)
• A raise statement evaluates an expression to produce an exception and circumvents the normal execution order. It is identified by the raise keyword.
• A break statement terminates the currently active loop. It is identified (and consists solely of) the break keyword.
• A continue statement skips the rest of the body of the currently active loop an attempts to start a new iteration. It is identified (and consists solely of) the continue keyword.
• An import statement potentially defines a new module object and binds one or more names in the current scope to either a module or values defined in the module. (Like the class statement, there are various hooks available to override exactly what happens during the execution of an import statement.) There are several forms, all of which share the import keyword.
• A global and nonlocal statement alters the scope in which an assignment to a particular name operates. They are identified by the global and nonlocal keywords, respectively.
• An if statement evaluates a boolean expression to select a set of statements to execute next. It is identified by the if keyword.
• A while statement evaluates a boolean expression to determine whether to execute its body, repeating the process until the expression become false. It is identified by the while keyword`.
• A for loop evaluates an expression to produce an iterator, which it uses to perform some name bindings and repeatedly execute a set of statements with those bindings. It is identified by the for keyword (which is also shared by generator expressions and comprehensions, though context makes it clear which is which).
• A try statement executes a set of statements and catches any exceptions those statements might raise. It is identified by the try keyword.
• A with statement evaluates an expression to produce a context manager, which provides a pair of functions to call before and after a set of statements is executed. It is identified by the with keyword.
• A function definition produces a callable object that wraps one or more statements to be execute when the object is called. It is identified by the def keyword.
• A class statement evaluates a set of statements to produce a set of bindings to be used as attributes of a new type. (Like an import statement, there are various hooks to override exactly what happens during the execution of a class statement.) It is identified by the class keyword.
• Coroutines are produced by various function, for, and with statements using the async keyword to distinguish them from their synchronous counterparts.