What s the difference between eval exec and compile

Question

I ve been looking at dynamic evaluation of Python code  and come across the eval   and compile   functions  and the exec statement   Can someone please explain the difference between eval and exec  and how the different modes of compile   fit in

User · Answer

exec is not an expression  a statement in Python 2 x  and a function in Python 3 x  It compiles and immediately evaluates a statement or set of statement contained in a string  Example   exec  print 5                prints 5     exec  print 5      if you use Python 2 x  nor the exec neither the print is a function there  exec  print 5  nprint 6       prints 5 newline 6   exec  if True  print 6       prints 6   exec  5                     does nothing and returns nothing    eval is a built-in function  not a statement   which evaluates an expression and returns the value that expression produces  Example   x   eval  5                  x  lt - 5  x   eval   d   6    x        x  lt - 11  x   eval  abs  d     -100    x  lt - 100  x   eval  x   5              INVALID  assignment is not an expression   x   eval  if 1  x   4        INVALID  if is a statement  not an expression    compile is a lower level version of exec and eval  It does not execute or evaluate your statements or expressions  but returns a code object that can do it  The modes are as follows   compile string       eval   returns the code object that would have been executed had you done eval string   Note that you cannot use statements in this mode  only a  single  expression is valid   compile string       exec   returns the code object that would have been executed had you done exec string   You can use any number of statements here   compile string       single   is like the exec mode but expects exactly one expression statement  eg compile  a 1 if 1 else 3    myf   mode  single

User · Answer

The short answer  or TL DR  Basically  eval is used to evaluate a single dynamically generated Python expression  and exec is used to execute dynamically generated Python code only for its side effects   eval and exec have these two differences    eval accepts only a single expression  exec can take a code block that has Python statements  loops  try  except   class and function method definitions and so on    An expression in Python is whatever you can have as the value in a variable assignment   a variable    anything you can put within these parentheses is an expression   eval returns the value of the given expression  whereas exec ignores the return value from its code  and always returns None  in Python 2 it is a statement and cannot be used as an expression  so it really does not return anything     In versions 1 0 - 2 7  exec was a statement  because CPython needed to produce a different kind of code object for functions that used exec for its side effects inside the function    In Python 3  exec is a function  its use has no effect on the compiled bytecode of the function where it is used     Thus basically    gt  gt  gt  a   5  gt  gt  gt  eval  37   a       it is an expression 42  gt  gt  gt  exec  37   a       it is an expression statement  value is ignored  None is returned   gt  gt  gt  exec  a   47       modify a global variable as a side effect  gt  gt  gt  a 47  gt  gt  gt  eval  a   47      you cannot evaluate a statement Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt    File   lt string gt    line 1     a   47         SyntaxError  invalid syntax     The compile in  exec  mode compiles any number of statements into a bytecode that implicitly always returns None  whereas in  eval  mode it compiles a single expression into bytecode that returns the value of that expression     gt  gt  gt  eval compile  42     lt string gt     exec       code returns None  gt  gt  gt  eval compile  42     lt string gt     eval       code returns 42 42  gt  gt  gt  exec compile  42     lt string gt     eval       code returns 42   gt  gt  gt                                             but ignored by exec   In the  eval  mode  and thus with the eval function if a string is passed in   the compile raises an exception if the source code contains statements or anything else beyond a single expression    gt  gt  gt  compile  for i in range 3   print i      lt string gt     eval   Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt    File   lt string gt    line 1     for i in range 3   print i          SyntaxError  invalid syntax     Actually the statement  eval accepts only a single expression  applies only when a string  which contains Python source code  is passed to eval  Then it is internally compiled to bytecode using compile source    lt string gt     eval   This is where the difference really comes from   If a code object  which contains Python bytecode  is passed to exec or eval  they behave identically  excepting for the fact that exec ignores the return value  still returning None always  So it is possible use eval to execute something that has statements  if you just compiled it into bytecode before instead of passing it as a string    gt  gt  gt  eval compile  if 1  print  Hello       lt string gt     exec    Hello  gt  gt  gt    works without problems  even though the compiled code contains statements  It still returns None  because that is the return value of the code object returned from compile   In the  eval  mode  and thus with the eval function if a string is passed in   the compile raises an exception if the source code contains statements or anything else beyond a single expression    gt  gt  gt  compile  for i in range 3   print i      lt string gt     eval   Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt    File   lt string gt    line 1     for i in range 3   print i          SyntaxError  invalid syntax   The longer answer  a k a the gory details  exec and eval  The exec function  which was a statement in Python 2  is used for executing a dynamically created statement or program    gt  gt  gt  program       for i in range 3       print  Python is cool        gt  gt  gt  exec program  Python is cool Python is cool Python is cool  gt  gt  gt     The eval function does the same for a single expression  and returns the value of the expression    gt  gt  gt  a   2  gt  gt  gt  my calculation    42   a   gt  gt  gt  result   eval my calculation   gt  gt  gt  result 84   exec and eval both accept the program expression to be run either as a str  unicode or bytes object containing source code  or as a code object which contains Python bytecode    If a str unicode bytes containing source code was passed to exec  it behaves equivalently to   exec compile source    lt string gt     exec      and eval similarly behaves equivalent to   eval compile source    lt string gt     eval        Since all expressions can be used as statements in Python  these are called the Expr nodes in the Python abstract grammar  the opposite is not true   you can always use exec if you do not need the return value  That is to say  you can use either eval  my func 42    or exec  my func 42     the difference being that eval returns the value returned by my func  and exec discards it    gt  gt  gt  def my func arg           print  Called with  d    arg          return arg   2       gt  gt  gt  exec  my func 42    Called with 42  gt  gt  gt  eval  my func 42    Called with 42 84  gt  gt  gt     Of the 2  only exec accepts source code that contains statements  like def  for  while  import  or class  the assignment statement  a k a a   42   or entire programs    gt  gt  gt  exec  for i in range 3   print i    0 1 2  gt  gt  gt  eval  for i in range 3   print i    Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt    File   lt string gt    line 1     for i in range 3   print i          SyntaxError  invalid syntax     Both exec and eval accept 2 additional positional arguments - globals and locals - which are the global and local variable scopes that the code sees  These default to the globals   and locals   within the scope that called exec or eval  but any dictionary can be used for globals and any mapping for locals  including dict of course   These can be used not only to restrict modify the variables that the code sees  but are often also used for capturing the variables that the executed code creates    gt  gt  gt  g   dict    gt  gt  gt  l   dict    gt  gt  gt  exec  global a  a  b   123  42   g  l   gt  gt  gt  g  a   123  gt  gt  gt  l   b   42     If you display the value of the entire g  it would be much longer  because exec and eval add the built-ins module as   builtins   to the globals automatically if it is missing    In Python 2  the official syntax for the exec statement is actually exec code in globals  locals  as in   gt  gt  gt  exec  global a  a  b   123  42  in g  l   However the alternate syntax exec code  globals  locals  has always been accepted too  see below    compile  The compile source  filename  mode  flags 0  dont inherit False  optimize -1  built-in can be used to speed up repeated invocations of the same code with exec or eval by compiling the source into a code object beforehand  The mode parameter controls the kind of code fragment the compile function accepts and the kind of bytecode it produces  The choices are  eval    exec  and  single      eval  mode expects a single expression  and will produce bytecode that when run will return the value of that expression    gt  gt  gt  dis dis compile  a   b     lt string gt     eval      1           0 LOAD NAME                0  a                3 LOAD NAME                1  b                6 BINARY ADD               7 RETURN VALUE   exec  accepts any kinds of python constructs from single expressions to whole modules of code  and executes them as if they were module top-level statements  The code object returns None    gt  gt  gt  dis dis compile  a   b     lt string gt     exec      1           0 LOAD NAME                0  a                3 LOAD NAME                1  b                6 BINARY ADD               7 POP TOP                              lt - discard result               8 LOAD CONST               0  None     lt - load None on stack              11 RETURN VALUE                         lt - return top of stack   single  is a limited form of  exec  which accepts a source code containing a single statement  or multiple statements separated by    if the last statement is an expression statement  the resulting bytecode also prints the repr of the value of that expression to the standard output      An if-elif-else chain  a loop with else  and try with its except  else and finally blocks is considered a single statement   A source fragment containing 2 top-level statements is an error for the  single   except in Python 2 there is a bug that sometimes allows multiple toplevel statements in the code  only the first is compiled  the rest are ignored   In Python 2 7 8    gt  gt  gt  exec compile  a   5 na   6     lt string gt     single     gt  gt  gt  a 5   And in Python 3 4 2    gt  gt  gt  exec compile  a   5 na   6     lt string gt     single    Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt    File   lt string gt    line 1     a   5           SyntaxError  multiple statements found while compiling a single statement   This is very useful for making interactive Python shells  However  the value of the expression is not returned  even if you eval the resulting code    Thus greatest distinction of exec and eval actually comes from the compile function and its modes     In addition to compiling source code to bytecode  compile supports compiling abstract syntax trees  parse trees of Python code  into code objects  and source code into abstract syntax trees  the ast parse is written in Python and just calls compile source  filename  mode  PyCF ONLY AST    these are used for example for modifying source code on the fly  and also for dynamic code creation  as it is often easier to handle the code as a tree of nodes instead of lines of text in complex cases     While eval only allows you to evaluate a string that contains a single expression  you can eval a whole statement  or even a whole module that has been compiled into bytecode  that is  with Python 2  print is a statement  and cannot be evalled directly    gt  gt  gt  eval  for i in range 3   print  Python is cool     Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt    File   lt string gt    line 1     for i in range 3   print  Python is cool           SyntaxError  invalid syntax   compile it with  exec  mode into a code object and you can eval  it  the eval function will return None    gt  gt  gt  code   compile  for i in range 3   print  Python is cool                         foo py    exec    gt  gt  gt  eval code  Python is cool Python is cool Python is cool   If one looks into eval and exec source code in CPython 3  this is very evident  they both call PyEval EvalCode with same arguments  the only difference being that exec explicitly returns None   Syntax differences of exec between Python 2 and Python 3  One of the major differences in Python 2 is that exec is a statement and eval is a built-in function  both are built-in functions in Python 3   It is a well-known fact that the official syntax of exec in Python 2 is exec code  in globals   locals     Unlike majority of the Python 2-to-3 porting guides seem to suggest  the exec statement in CPython 2 can be also used with syntax that looks exactly like the exec function invocation in Python 3  The reason is that Python 0 9 9 had the exec code  globals  locals  built-in function  And that built-in function was replaced with exec statement somewhere before Python 1 0 release   Since it was desirable to not break backwards compatibility with Python 0 9 9  Guido van Rossum added a compatibility hack in 1993  if the code was a tuple of length 2 or 3  and globals and locals were not passed into the exec statement otherwise  the code would be interpreted as if the 2nd and 3rd element of the tuple were the globals and locals respectively  The compatibility hack was not mentioned even in Python 1 4 documentation  the earliest available version online   and thus was not known to many writers of the porting guides and tools  until it was documented again in November 2012      The first expression may also be a tuple of length 2 or 3  In this case  the optional parts must be omitted  The form exec expr  globals  is equivalent to exec expr in globals  while the form exec expr  globals  locals  is equivalent to exec expr in globals  locals  The tuple form of exec provides compatibility with Python 3  where exec is a function rather than a statement    Yes  in CPython 2 7 that it is handily referred to as being a forward-compatibility option  why confuse people over that there is a backward compatibility option at all    when it actually had been there for backward-compatibility for two decades   Thus while exec is a statement in Python 1 and Python 2  and a built-in function in Python 3 and Python 0 9 9    gt  gt  gt  exec  print a    globals      a   42   42   has had identical behaviour in possibly every widely released Python version ever  and works in Jython 2 5 2  PyPy 2 3 1  Python 2 7 6  and IronPython 2 6 1 too  kudos to them following the undocumented behaviour of CPython closely    What you cannot do in Pythons 1 0 - 2 7 with its compatibility hack  is to store the return value of exec into a variable   Python 2 7 11   default  Apr 17 2016  14 00 29    GCC 5 3 1 20160413  on linux2 Type  help    copyright    credits  or  license  for more information   gt  gt  gt  a   exec  print 42      File   lt stdin gt    line 1     a   exec  print 42                 SyntaxError  invalid syntax    which wouldn t be useful in Python 3 either  as exec always returns None   or pass a reference to exec    gt  gt  gt  call later exec   print 42    delay 1000    File   lt stdin gt    line 1     call later exec   print 42    delay 1000                      SyntaxError  invalid syntax   Which a pattern that someone might actually have used  though unlikely    Or use it in a list comprehension    gt  gt  gt   exec i  for i in   print 42     print foo      File   lt stdin gt    line 1      exec i  for i in   print 42     print foo              SyntaxError  invalid syntax   which is abuse of list comprehensions  use a for loop instead

User · Answer

exec is for statement and does not return anything  eval is for expression and returns value of expression   expression means  something  while statement means  do something

[python] What's the difference between eval, exec, and compile?

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