What is the use of assert

Question

I have been reading some source code and in several places I have seen the usage of assert    What does it mean exactly  What is its usage

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Others have already given you links to documentation.

You can try the following in a interactive shell:

>>> assert 5 > 2
>>> assert 2 > 5
Traceback (most recent call last):
  File "<string>", line 1, in <fragment>
builtins.AssertionError:

The first statement does nothing, while the second raises an exception. This is the first hint: asserts are useful to check conditions that should be true in a given position of your code (usually, the beginning (preconditions) and the end of a function (postconditions)).

Asserts are actually highly tied to programming by contract, which is a very useful engineering practice:

http://en.wikipedia.org/wiki/Design_by_contract.

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Python assert is basically a debugging aid which test condition for internal self-check of your code  Assert makes debugging really easy when your code gets into impossible edge cases  Assert check those impossible cases   Let s say there is a function to calculate price of item after discount    def calculate discount price  discount       discounted price   price -  discount price      assert 0  lt   discounted price  lt   price     return discounted price   here  discounted price can never be less than 0 and greater than actual price  So  in case the above condition is violated assert raises an Assertion Error  which helps the developer to identify that something impossible had happened   Hope it helps

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Assertions are statements that state a fact confidently in our program  Syntax     assert  lt condition gt  or assert  lt condition gt   lt error message gt  It has a condition expression which is supposed to be always true  If the condition is false  the assert statement will halt the program and throw an error message saying AssertionError  So your assertion expression will be something that you don t want in your program  e g   assert  lt condition gt  -- using assert without  lt error message gt  var   int input  quot Enter value 1-9 inclusive  quot                                     assert var  0  print var   Output   If input is 0   AssertionError  If input is 1   1   assert  lt condition gt   lt error message gt  -- using assert with an  lt error message gt  var   int input  quot Enter value 1-9 inclusive  quot                                     assert var  0  quot Input cannot be zero quot  print var   Output   If input is 0   AssertionError  Input cannot be zero  If input is 1   1    Key Points    It is used as a debugging tool  It takes an expression and an optional message  It exists in almost every programming language

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if the statement after assert  is true then the program continues   but if the statement after assert is false then the program gives an error  Simple as that   e g    assert 1 gt 0    normal execution assert 0 gt 1    Traceback  most recent call last                 File   lt pyshell 11 gt    line 1  in  lt module gt                assert 0 gt 1               AssertionError

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My short explanation is    assert raises AssertionError if expression is false  otherwise just continues the code  and if there s a comma whatever it is it will be AssertionError  whatever after comma  and to code is like  raise AssertionError whatever after comma    A related tutorial about this      https   www tutorialspoint com python assertions in python htm

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In Pycharm  if you use assert along with isinstance to declare an object s type  it will let you access the methods and attributes of the parent object while you are coding  it will auto-complete automatically   For example  let s say self object1 object2 is a MyClass object   import MyClasss  def code it self       testObject   self object1 object2   at this point  program doesn t know that testObject  is a MyClass object yet     assert isinstance testObject   MyClasss    now the program knows testObject is a MyClass object     testObject do it     from this point on  PyCharm will be able to auto-complete when you are working on testObject

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If you ever want to know exactly what a reserved function does in python  type in help enter keyword   Make sure if you are entering a reserved keyword that you enter it as a string

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format     assert Expression  arguments  When assert encounters a statement Python evaluates the expression If the statement is not true an exception is raised assertionError   If the assertion fails  Python uses ArgumentExpression as the argument for the AssertionError  AssertionError exceptions can be caught and handled like any other exception using the try-except statement  but if not handled  they will terminate the program and produce a traceback  Example   def KelvinToFahrenheit Temperature           assert  Temperature  gt   0   Colder than absolute zero           return   Temperature-273  1 8  32     print KelvinToFahrenheit 273      print int KelvinToFahrenheit 505 78       print KelvinToFahrenheit -5        When the above code is executed  it produces the following result   32 0 451 Traceback  most recent call last         File  test py   line 9  in  lt module gt          print KelvinToFahrenheit -5        File  test py   line 4  in KelvinToFahrenheit         assert  Temperature  gt   0   Colder than absolute zero       AssertionError  Colder than absolute zero

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The assert statement exists in almost every programming language  It helps detect problems early in your program  where the cause is clear  rather than later when some other operation fails  When you do    assert condition      you re telling the program to test that condition  and immediately trigger an error if the condition is false  In Python  it s roughly equivalent to this  if not condition      raise AssertionError    Try it in the Python shell   gt  gt  gt  assert True   nothing happens  gt  gt  gt  assert False Traceback  most recent call last     File  quot  lt stdin gt  quot   line 1  in  lt module gt  AssertionError  Assertions can include an optional message  and you can disable them when running the interpreter  To print a message if the assertion fails  assert False   quot Oh no  This assertion failed  quot   Do not use parenthesis to call assert like a function  It is a statement  If you do assert condition  message  you ll be running the assert with a  condition  message  tuple as first parameter  As for disabling them  when running python in optimized mode  where   debug   is False  assert statements will be ignored  Just pass the -O flag  python -O script py  See here for the relevant documentation

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gt  gt  gt this is very complex function result   9  gt  gt  gt c   this is very complex function result  gt  gt  gt test us    c  lt  4    gt  gt  gt   first we try without assert  gt  gt  gt if test us    True      print  YES  I am right    else      print  I am Wrong  but the program still RUNS     I am Wrong  but the program still RUNS     gt  gt  gt   now we try with assert  gt  gt  gt  assert test us Traceback  most recent call last     File   lt pyshell 52 gt    line 1  in  lt module gt      assert test us AssertionError  gt  gt  gt

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As written in other answers  assert statements are used to check the state of the program at a given point   I won t repeat what was said about associated message  parentheses  or -O option and   debug   constant  Check also the doc for first hand information  I will focus on your question  what is the use of assert  More precisely  when  and when not  should one use assert   The assert statements are useful to debug a program  but discouraged to check user input  I use the following rule of thumb  keep assertions to detect a this should not happen situation  A user input may be incorrect  e g  a password too short  but this is not a this should not happen case  If the diameter of a circle is not twice as large as its radius  you are in a this should not happen case   The most interesting  in my mind  use of assert is inspired by the programming by contract as described by B  Meyer in  Object-Oriented Software Construction   https   www eiffel org doc eiffel Object-Oriented Software Construction 2C 2nd Edition   and implemented in the  Eiffel programming language   https   en wikipedia org wiki Eiffel  programming language    You can t fully emulate programming by contract using the assert statement  but it s interesting to keep the intent   Here s an example  Imagine you have to write a head function  like the  head function in Haskell   http   www zvon org other haskell Outputprelude head f html    The specification you are given is   if the list is not empty  return the first item of a list   Look at the following implementations    gt  gt  gt  def head1 xs   return xs 0    And   gt  gt  gt  def head2 xs           if len xs   gt  0              return xs 0          else              return None    Yes  this can be written as return xs 0  if xs else None  but that s not the point    If the list is not empty  both functions have the same result and this result is correct    gt  gt  gt  head1  1  2  3      head2  1  2  3      1 True   Hence  both implementations are  I hope  correct  They differ when you try to take the head item of an empty list    gt  gt  gt  head1     Traceback  most recent call last       IndexError  list index out of range   But    gt  gt  gt  head2     is None True   Again  both implementations are correct  because no one should pass an empty list to these functions  we are out of the specification   That s an incorrect call  but if you do such a call  anything can happen  One function raises an exception  the other returns a special value  The most important is  we can t rely on this behavior  If xs is empty  this will work   print head2 xs     But this will crash the program   print head1 xs     To avoid some surprises  I would like to know when I m passing some unexpected argument to a function  In other words  I would like to know when the observable behavior is not reliable  because it depends on the implementation  not on the specification  Of course  I can read the specification  but programmers do not always read carefully the docs   Imagine if I had a way to insert the specification into the code to get the following effect  when I violate the specification  e g by passing an empty list to head  I get a warning  That would be a great help to write a correct  i e  compliant with the specification  program  And that s where assert enters on the scene    gt  gt  gt  def head1 xs           assert len xs   gt  0   The list must not be empty          return xs 0    And   gt  gt  gt  def head2 xs           assert len xs   gt  0   The list must not be empty          if len xs   gt  0              return xs 0          else              return None   Now  we have    gt  gt  gt  head1     Traceback  most recent call last       AssertionError  The list must not be empty   And    gt  gt  gt  head2     Traceback  most recent call last       AssertionError  The list must not be empty   Note that head1 throws an AssertionError  not an IndexError  That s important because an AssertionError is not any runtime error  it signals a violation of the specification  I wanted a warning  but I get an error  Fortunately  I can disable the check  using the -O option   but at my own risks  I will do it a crash is really expensive  and hope for the best  Imagine my program is embedded in a spaceship that travels through a black hole  I will disable assertions and hope the program is robust enough to not crash as long as possible   This example was only about preconditions  be you can use assert to check postconditions  the return value and or the state  and invariants  state of a class   Note that checking postconditions and invariants with assert can be cumbersome    for postconditions  you need to assign the return value to a variable  and maybe to store the iniial state of the object if you are dealing with a method  for invariants  you have to check the state before and after a method call    You won t have something as sophisticated as Eiffel  but you can however improve the overall quality of a program     To summarize  the assert statement is a convenient way to detect a this should not happen situation  Violations of the specification  e g  passing an empty list to head  are first class this should not happen situations  Hence  while the assert statement may be used to detect any unexpected situation  it is a privilegied way to ensure that the specification is fulfilled  Once you have inserted assert statements into the code to represent the specification  we can hope you have improved the quality of the program because incorrect arguments  incorrect return values  incorrect states of a class     will be reported

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Watch out for the parentheses  As has been pointed out above  in Python 3  assert is still a statement  so by analogy with print      one may extrapolate the same to assert     or raise     but you shouldn t   This is important because   assert 2   2    5   Houston we ve got a problem     won t work  unlike  assert 2   2    5   Houston we ve got a problem    The reason the first one will not work is that bool   False   Houston we ve got a problem     evaluates to True   In the statement assert False   these are just redundant parentheses around False  which evaluate to their contents  But with assert False   the parentheses are now a tuple  and a non-empty tuple evaluates to True in a boolean context

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As summarized concisely on the C2 Wiki      An assertion is a boolean expression at a specific point in a program which will be true unless there is a bug in the program    You can use an assert statement to document your understanding of the code at a particular program point  For example  you can document assumptions or guarantees about inputs  preconditions   program state  invariants   or outputs  postconditions    Should your assertion ever fail  this is an alert for you  or your successor  that your understanding of the program was wrong when you wrote it  and that it likely contains a bug   For more information  John Regehr has a wonderful blog post on the Use of Assertions  which applies to the Python assert statement as well

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def getUser self  id  Email        user key   id and id or Email      assert user key   Can be used to ensure parameters are passed in the function call

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Here is a simple example  save this in file  let s say b py   def chkassert num       assert type num     int   chkassert  a     and the result when  python b py  Traceback  most recent call last     File  b py   line 5  in  lt module gt      chkassert  a     File  b py   line 2  in chkassert     assert type num     int AssertionError

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As other answers have noted  assert is similar to throwing an exception if a given condition isn t true  An important difference is that assert statements get ignored if you compile your code with the optimization option -O  The documentation says that assert expression can better be described as being equivalent to   if   debug       if not expression  raise AssertionError   This can be useful if you want to thoroughly test your code  then release an optimized version when you re happy that none of your assertion cases fail - when optimization is on  the   debug   variable becomes False and the conditions will stop getting evaluated  This feature can also catch you out if you re relying on the asserts and don t realize they ve disappeared

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The assert statement has two forms   The simple form  assert  lt expression gt   is equivalent to  if    debug        if not  lt expression gt   raise AssertionError   The extended form  assert  lt expression1 gt    lt expression2 gt   is equivalent to  if    debug        if not  lt expression1 gt   raise AssertionError   lt expression2 gt

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The goal of an assertion in Python is to inform developers about unrecoverable errors in a program   Assertions are not intended to signal expected error conditions  like    file not found     where a user can take corrective action  or just try again    Another way to look at it is to say that assertions are internal self-checks in your code  They work by declaring some conditions as impossible in your code  If these conditions don   t hold that means there   s a bug in the program   If your program is bug-free  these conditions will never occur  But if one of them does occur the program will crash with an assertion error telling you exactly which    impossible    condition was triggered  This makes it much easier to track down and fix bugs in your programs   Here   s a summary from a tutorial on Python   s assertions I wrote      Python   s assert statement is a debugging aid  not a mechanism for handling run-time errors  The goal of using assertions is to let developers find the likely root cause of a bug more quickly  An assertion error should never be raised unless there   s a bug in your program

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The assert statement exists in almost every programming language  It helps detect problems early in your program  where the cause is clear  rather than later as a side-effect of some other operation  They always expect a True condition   When you do something like   assert condition   You re telling the program to test that condition and immediately trigger an error if it is false   In Python  assert expression  is equivalent to   if   debug        if not  lt expression gt   raise AssertionError   You can use the extended expression to pass an optional message   if   debug        if not  expression 1   raise AssertionError expression 2    Try it in the Python interpreter    gt  gt  gt  assert True   Nothing happens because the condition returns a True value   gt  gt  gt  assert False   A traceback is triggered because this evaluation did not yield an expected value  Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  AssertionError   There are some caveats to seen before using them mainly for those who deem to toggles between the assert and if statements  The aim to use assert is on occasions when the program verifies a condition and return a value that should stop the program immediately instead of taking some alternative way to bypass the error   1  Parentheses  As you may have noticed  the assert statement uses two conditions  Hence  do not use parentheses to englobe them as one for obvious advice  If you do such as   assert  condition  message    Example    gt  gt  gt  assert  1  2  1  1   lt stdin gt  1  SyntaxWarning  assertion is always true  perhaps remove parentheses    You will be running the assert with a  condition  message  which represents a tuple as the first parameter  and this happens cause non-empty tuple in Python is always True  However  you can do separately without problem   assert  condition    message    Example    gt  gt  gt  assert  1  2     This condition returns a  s value       False  Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  AssertionError  This condition returns a False value    2  Debug purpose  If you are wondering regarding when use assert statement  Take an example used in real life     When your program tends to control each parameter entered by the user or whatever else   def loremipsum   kwargs       kwargs pop  bar     return 0 if  bar  isn t in parameter     kwargs setdefault  foo   type self     returns  type self   value by default     assert  len kwargs     0    unrecognized parameter passed in  s         join kwargs keys        Another case is on math when 0 or non-positive as a coefficient or constant on a certain equation   def discount item  percent       price   int item  price      1 0 - percent       print price      assert  0  lt   price  lt   item  price                   Discounted prices cannot be lower than 0                 and they cannot be higher than the original price        return price     or even a simple example of a boolean implementation   def true a  b       assert  a    b    False      return 1  def false a  b       assert  a    b    True      return 0   3  Data processing or data validation  The utmost importance is to not rely on the assert statement to execute data processing or data validation because this statement can be turned off on the Python initialization with -O or -OO flag     meaning value 1  2  and 0  as default   respectively     or PYTHONOPTIMIZE environment variable   Value 1     asserts are disabled     bytecode files are generated using  pyo extension instead of  pyc     sys flags optimize is set to 1  True      and    debug   is set to False   Value 2  disables one more stuff    docstrings are disabled   Therefore  using the assert statement to validate a sort of expected data is extremely dangerous  implying even to some security issues  Then  if you need to validate some permission I recommend you raise AuthError instead  As a preconditional effective  an assert is commonly used by programmers on libraries or modules that do not have a user interact directly

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Basically the assert keyword meaning is that if the condition is not true then it through an assertionerror else it continue for example in python   code-1  a 5  b 6  assert a  b   OUTPUT    assert a  b  AssertionError   code-2  a 5  b 5  assert a  b   OUTPUT    Process finished with exit code 0

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Assertions are a systematic way to check that the internal state of a program is as the programmer expected  with the goal of catching bugs  See the example below    gt  gt  gt  number   input  Enter a positive number    Enter a positive number -1  gt  gt  gt  assert  number  gt  0    Only positive numbers are allowed   Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  AssertionError  Only positive numbers are allowed   gt  gt  gt

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From docs      Assert statements are a convenient way to insert debugging assertions into a program   You can read more here  http   docs python org release 2 5 2 ref assert html

[python] What is the use of "assert"?

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