When to use RSpec let

Question

I tend to use before blocks to set instance variables  I then use those variables across my examples   I recently came upon let    According to RSpec docs  it is used to         to define a memoized helper method  The value will be cached across multiple calls in the same example but not across examples    How is this different from using instance variables in before blocks  And also when should you use let   vs before

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I always prefer let to an instance variable for a couple of reasons    Instance variables spring into existence when referenced   This means that if you fat finger the spelling of the instance variable  a new one will be created and initialized to nil  which can lead to subtle bugs and false positives   Since let creates a method  you ll get a NameError when you misspell it  which I find preferable   It makes it easier to refactor specs  too  A before  each  hook will run before each example  even if the example doesn t use any of the instance variables defined in the hook   This isn t usually a big deal  but if the setup of the instance variable takes a long time  then you re wasting cycles   For the method defined by let  the initialization code only runs if the example calls it  You can refactor from a local variable in an example directly into a let without changing the referencing syntax in the example   If you refactor to an instance variable  you have to change how you reference the object in the example  e g  add an     This is a bit subjective  but as Mike Lewis pointed out  I think it makes the spec easier to read   I like the organization of defining all my dependent objects with let and keeping my it block nice and short    A related link can be found here  http   www betterspecs org  let

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I use let to test my HTTP 404 responses in my API specs using contexts   To create the resource  I use let   But to store the resource identifier  I use let  Take a look how it looks like   let   country      create  country    let  country id    country id   before             get  api countries   country id      it  responds with HTTP 200    should respond with 200     context  when the country does not exist  do   let  country id    -1     it  responds with HTTP 404    should respond with 404    end   That keeps the specs clean and readable

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In general  let   is a nicer syntax  and it saves you typing  name symbols all over the place  But  caveat emptor  I have found let   also introduces subtle bugs  or at least head scratching  because the variable doesn t really exist until you try to use it    Tell tale sign  if adding a puts after the let   to see that the variable is correct allows a spec to pass  but without the puts the spec fails -- you have found this subtlety   I have also found that let   doesn t seem to cache in all circumstances  I wrote it up in my blog  http   technicaldebt com  p 1242  Maybe it is just me

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before  by default implies before  each   Ref The Rspec Book  copyright 2010  page 228   before scope    each  options      amp block    I use before  each  to seed some data for each example group without having to call the let method to create the data in the  it  block  Less code in the  it  block in this case   I use let if I want some data in some examples but not others    Both before and let are great for DRYing up the  it  blocks   To avoid any confusion   let  is not the same as before  all    Let  re-evaluates its method and value for each example   it    but caches the value across multiple calls in the same example  You can read more about it here  https   www relishapp com rspec rspec-core v 2-6 docs helper-methods let-and-let

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Dissenting voice here  after 5 years of rspec I don t like let very much    1  Lazy evaluation often makes test setup confusing  It becomes difficult to reason about setup when some things that have been declared in setup are not actually affecting state  while others are   Eventually  out of frustration someone just changes let to let   same thing without lazy evaluation  in order to get their spec working  If this works out for them  a new habit is born  when a new spec is added to an older suite and it doesn t work  the first thing the writer tries is to add bangs to random let calls   Pretty soon all the performance benefits are gone   2  Special syntax is unusual to non-rspec users  I would rather teach Ruby to my team than the tricks of rspec  Instance variables or method calls are useful everywhere in this project and others  let syntax will only be useful in rspec   3  The  benefits  allow us to easily ignore good design changes  let   is good for expensive dependencies that we don t want to create over and over   It also pairs well with subject  allowing you to dry up repeated calls to multi-argument methods  Expensive dependencies repeated in many times  and methods with big signatures are both points where we could make the code better    maybe I can introduce a new abstraction that isolates a dependency from the rest of my code  which would mean fewer tests need it  maybe the code under test is doing too much maybe I need to inject smarter objects instead of a long list of primitives maybe I have a violation of tell-don t-ask maybe the expensive code can be made faster  rarer - beware of premature optimisation here    In all these cases  I can address the symptom of difficult tests with a soothing balm of rspec magic  or I can try address the cause  I feel like I spent way too much of the last few years on the former and now I want some better code   To answer the original question  I would prefer not to  but I do still use let  I mostly use it to fit in with the style of the rest of the team  it seems like most Rails programmers in the world are now deep into their rspec magic so that is very often   Sometimes I use it when I m adding a test to some code that I don t have control of  or don t have time to refactor to a better abstraction  i e  when the only option is the painkiller

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Note to Joseph -- if you are creating database objects in a before  all  they won t be captured in a transaction and you re much more likely to leave cruft in your test database   Use before  each  instead   The other reason to use let and its lazy evaluation is so you can take a complicated object and test individual pieces by overriding lets in contexts  as in this very contrived example   context  foo  do   let  params  do         foo   gt  foo    bar   gt   bar      end   let  foo     foo      it  is set to foo  do     params  foo  should eq  foo     end   context  when foo is bar  do     let  foo     bar          NOTE we didn t have to redefine params entirely      it  is set to bar  do       params  foo  should eq  bar       end   end end

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The difference between using instances variables and let   is that let   is lazy-evaluated  This means that let   is not evaluated until the method that it defines is run for the first time   The difference between before and let is that let   gives you a nice way of defining a group of variables in a  cascading  style  By doing this  the spec looks a little better by simplifying the code

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It is important to keep in mind that let is lazy evaluated and not putting side-effect methods in it otherwise you would not be able to change from let to before  each  easily  You can use let  instead of let so that it is evaluated before each scenario

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I have completely replaced all uses of instance variables in my rspec tests to use let    I ve written a quickie example for a friend who used it to teach a small Rspec class  http   ruby-lambda blogspot com 2011 02 agile-rspec-with-let html  As some of the other answers here says  let   is lazy evaluated so it will only load the ones that require loading  It DRYs up the spec and make it more readable  I ve in fact ported the Rspec let   code to use in my controllers  in the style of inherited resource gem  http   ruby-lambda blogspot com 2010 06 stealing-let-from-rspec html  Along with lazy evaluation  the other advantage is that  combined with ActiveSupport  Concern  and the load-everything-in spec support  behavior  you can create your very own spec mini-DSL specific to your application  I ve written ones for testing against Rack and RESTful resources    The strategy I use is Factory-everything  via Machinist Forgery Faker   However  it is possible to use it in combination with before  each  blocks to preload factories for an entire set of example groups  allowing the specs to run faster  http   makandra com notes 770-taking-advantage-of-rspec-s-let-in-before-blocks

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let is functional as its essentially a Proc  Also its cached    One gotcha I found right away with let    In a Spec block that is evaluating a change    let  object   FactoryGirl create  object   expect     post  destroy  id  review id   to change Object   count  by -1    You ll need to be sure to call let outside of your expect block  i e  you re calling FactoryGirl create in your let block  I usually do this by verifying the object is persisted    object persisted  should eq true   Otherwise when the let block is called the first time a change in the database will actually happen due to the lazy instantiation   Update  Just adding a note  Be careful playing code golf or in this case rspec golf with this answer    In this case  I just have to call some method to which the object responds  So I invoke the   persisted   method on the object as its truthy  All I m trying to do is instantiate the object  You could call empty  or nil  too  The point isn t the test but bringing the object ot life by calling it    So you can t refactor   object persisted  should eq true   to be  object should be persisted    as the object hasn t been instantiated    its lazy      Update 2  leverage the let  syntax for instant object creation  which should avoid this issue altogether  Note though it will defeat a lot of the purpose of the laziness of the non banged let   Also in some instances you might actually want to leverage the subject syntax instead of let as it may give you additional options   subject  object   FactoryGirl create  object

[ruby] When to use RSpec let()?

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