What is the difference between an expression and a statement in Python

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In Python  what is the difference between expressions and statements

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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

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An expression is something  while a statement does something  An expression is a statement as well  but it must have a return    gt  gt  gt  2   2           expression  gt  gt  gt  print 2   2       statement   PS The interpreter always prints out the values of all expressions

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Expressions    Expressions are formed by combining objects and operators  An expression has a value  which has a type  Syntax for a simple expression  lt object gt  lt operator gt  lt object gt    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

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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

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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 2pr    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    gt  gt  gt  23 23  gt  gt  gt  23l 23L  gt  gt  gt  range 4   0  1  2  3    gt  gt  gt  2L bin 2   0b100b10   gt  gt  gt  def func a          Statement  just part of the example           return a a       Statement          gt  gt  gt  func 3  4 36      gt  gt  gt  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 lt 0  x 0 but you can also have a conditional expression like x 0 if x lt 0 else 1 that are expressions  In other languages  like C  this form is called an operator like this x x lt 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

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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

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Though this isn t related to Python   An expression evaluates to a value  A statement does something    gt  gt  gt  x   2           an expression  gt  gt  gt  x   1           a statement   gt  gt  gt  y   x   1       a statement  gt  gt  gt  print y         a statement  in 2 x  2

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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   gt  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

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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  quot x quot       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

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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   gt  gt  gt  42 42  gt  gt  gt  n 17  gt  gt  gt  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   gt  gt  gt  n   17  gt  gt  gt  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   This can be useful - thinkpython2 by Allen B  Downey

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An expression is something that can be reduced to a value  for example  quot 1 3 quot  is an expression  but  quot foo   1 3 quot  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

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STATEMENT   A Statement is a action or a command that does something  Ex  If-Else Loops  etc  val a  Int   5 If a gt 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

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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

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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  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

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Expressions only contain identifiers  literals and operators  where operators include arithmetic and boolean operators  the function call operator    the subscription operator    and similar  and can be reduced to some kind of  value   which can be any Python object   Examples   3   5 map lambda x  x x  range 10    a x for a in some iterable  yield 7   Statements  see 1  2   on the other hand  are everything that can make up a line  or several lines  of Python code   Note that expressions are statements as well   Examples     all the above expressions print 42 if x  do y   return a   7

[python] What is the difference between an expression and a statement in Python?

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