{"id":3923,"date":"2020-09-03T00:54:56","date_gmt":"2020-09-02T19:24:56","guid":{"rendered":"https:\/\/golangbyexamples.com\/?p=3923"},"modified":"2020-09-04T01:06:01","modified_gmt":"2020-09-03T19:36:01","slug":"error-assertion-golang","status":"publish","type":"post","link":"https:\/\/golangbyexamples.com\/error-assertion-golang\/","title":{"rendered":"Get underlying type from error or error assertion in Go (Golang)"},"content":{"rendered":"\n<div id=\"ez-toc-container\" class=\"ez-toc-v2_0_17 counter-hierarchy counter-decimal ez-toc-light-blue\">\n<div class=\"ez-toc-title-container\">\n<p class=\"ez-toc-title\">Table of Contents<\/p>\n<span class=\"ez-toc-title-toggle\"><a class=\"ez-toc-pull-right ez-toc-btn ez-toc-btn-xs ez-toc-btn-default ez-toc-toggle\" style=\"display: none;\"><i class=\"ez-toc-glyphicon ez-toc-icon-toggle\"><\/i><\/a><\/span><\/div>\n<nav><ul class=\"ez-toc-list ez-toc-list-level-1\"><li class=\"ez-toc-page-1 ez-toc-heading-level-1\"><a class=\"ez-toc-link ez-toc-heading-1\" href=\"https:\/\/golangbyexamples.com\/error-assertion-golang\/#Overview\" title=\"Overview\">Overview<\/a><\/li><li class=\"ez-toc-page-1 ez-toc-heading-level-1\"><a class=\"ez-toc-link ez-toc-heading-2\" href=\"https:\/\/golangbyexamples.com\/error-assertion-golang\/#Code\" title=\"Code\">Code<\/a><\/li><\/ul><\/nav><\/div>\n<h1 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Overview\"><\/span><strong>Overview<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h1>\n\n\n\n<p class=\"has-medium-font-size\">There are two ways of getting the underlying type<\/p>\n\n\n\n<ul class=\"has-medium-font-size wp-block-list\"><li>Using the .({type}) assert<\/li><\/ul>\n\n\n\n<p class=\"has-medium-font-size\">If the assert succeeds then it will return the corresponding error otherwise it will panic. Below is the syntax<\/p>\n\n\n\n<pre class=\"wp-block-prismatic-blocks\"><code class=\"language-go\">err := err.({type})<\/code><\/pre>\n\n\n\n<p class=\"has-medium-font-size\">Better to use <strong>ok<\/strong> variable to prevent panic in case assertion fails.&nbsp; Below is the syntax for that. The <strong>ok<\/strong> variable will be set to true if&nbsp; error underlying type is correct<\/p>\n\n\n\n<pre class=\"wp-block-prismatic-blocks\"><code class=\"language-go\">err, ok := err.({type})<\/code><\/pre>\n\n\n\n<p class=\"has-medium-font-size\">Using the <strong>As<\/strong> function of <strong>errors<\/strong> package-&nbsp; <a href=\"https:\/\/golang.org\/pkg\/errors\/\">https:\/\/golang.org\/pkg\/errors\/<\/a> .Using <strong>As<\/strong> function is preferable to using the .({type}) assert because it checks for match by unwrapping the first error sequentially and matches it with target error at each step of unwrap. Below is the syntax of Is function<\/p>\n\n\n\n<pre class=\"wp-block-prismatic-blocks\"><code class=\"language-go\">func As(err error, target interface{}) bool<\/code><\/pre>\n\n\n\n<p class=\"has-medium-font-size\"><strong>As<\/strong> function will find the first error in the first argument which can match the target. Once a match is found it will set target to that error value.<\/p>\n\n\n\n<h1 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Code\"><\/span><strong>Code<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h1>\n\n\n\n<p class=\"has-medium-font-size\">&nbsp;Let&#8217;s see an example<\/p>\n\n\n\n<pre class=\"wp-block-prismatic-blocks\"><code class=\"language-go\">package main\n\nimport (\n\t\"errors\"\n\t\"fmt\"\n\t\"os\"\n)\n\nfunc main() {\n\n\terr := openFile(\"non-existing.txt\")\n\n\tif e, ok := err.(*os.PathError); ok {\n\t\tfmt.Printf(\"Using Assert: Error e is of type path error. Path: %v\\n\", e.Path)\n\t} else {\n\t\tfmt.Println(\"Using Assert: Error not of type path error\")\n\t}\n\n\tvar pathError *os.PathError\n\tif errors.As(err, &pathError) {\n\t\tfmt.Printf(\"Using As function: Error e is of type path error. Path: %v\\n\", pathError.Path)\n\t}\n}\n\nfunc openFile(fileName string) error {\n\t_, err := os.Open(\"non-existing.txt\")\n\tif err != nil {\n\t\treturn err\n\t}\n\treturn nil\n}<\/code><\/pre>\n\n\n\n<p class=\"has-medium-font-size\"><strong>Output:<\/strong><\/p>\n\n\n\n<pre class=\"wp-block-prismatic-blocks\"><code class=\"language-go\">Using Assert: Error e is of type path error. Path: non-existing.txt\nUsing As function: Error e is of type path error. Path: non-existing.txt<\/code><\/pre>\n\n\n\n<p class=\"has-medium-font-size\">In the above program, we have a function openFile in which we are trying to open a non-existing type hence it will raise an error. Then we are asserting&nbsp; the error in two ways<\/p>\n\n\n\n<ul class=\"has-medium-font-size wp-block-list\"><li>Using .assert operator. The ok variable will be set to true if the error underlying type is <strong>*os.PathError<\/strong> otherwise it will be set to false<\/li><\/ul>\n\n\n\n<pre class=\"wp-block-prismatic-blocks\"><code class=\"language-go\">e,ok := err.(*os.PathError); ok<\/code><\/pre>\n\n\n\n<ul class=\"has-medium-font-size wp-block-list\"><li>Using the <strong>As<\/strong> function of errors package<\/li><\/ul>\n\n\n\n<pre class=\"wp-block-prismatic-blocks\"><code class=\"language-go\">errors.As(err, &pathError)<\/code><\/pre>\n\n\n\n<p class=\"has-medium-font-size\">Both the method correctly assert that the error is of type <strong>*os.PathError<\/strong> as error returned by the <strong>openFile<\/strong> function is of type <strong>*os.PathError<\/strong><\/p>\n\n\n\n<p class=\"has-medium-font-size\">We mentioned above that using <strong>As<\/strong> the function is preferable to using the .({type}) assert because it checks for a match by unwrapping the first error sequentially and matches it with the target error at each step of unwrap. Let&#8217;s see an example to understand that<\/p>\n\n\n\n<pre class=\"wp-block-prismatic-blocks\"><code class=\"language-go\">import (\n\t\"errors\"\n\t\"fmt\"\n\t\"os\"\n)\n\nfunc main() {\n\tvar pathError *os.PathError\n\terr := openFile(\"non-existing.txt\")\n\n\tif e, ok := err.(*os.PathError); ok {\n\t\tfmt.Printf(\"Using Assert: Error e is of type path error. Error: %v\\n\", e)\n\t} else {\n\t\tfmt.Println(\"Using Assert: Error not of type path error\")\n\t}\n\n\tif errors.As(err, &pathError) {\n\t\tfmt.Printf(\"Using As function: Error e is of type path error. Error: %v\\n\", pathError)\n\t}\n}\n\nfunc openFile(fileName string) error {\n\t_, err := os.Open(\"non-existing.txt\")\n\tif err != nil {\n\t\treturn fmt.Errorf(\"Error opening: %w\", err)\n\t}\n\treturn nil\n}<\/code><\/pre>\n\n\n\n<p class=\"has-medium-font-size\"><strong>Output:<\/strong><\/p>\n\n\n\n<pre class=\"wp-block-prismatic-blocks\"><code class=\"language-go\">Using Assert: Error not of type path error\nUsing As function: Error e is of type path error. Error: open non-existing.txt: no such file or directory<\/code><\/pre>\n\n\n\n<p class=\"has-medium-font-size\">The above program is almost same as the previous program just the difference being that in the openFile function we are wrapping the error as well<\/p>\n\n\n\n<pre class=\"wp-block-prismatic-blocks\"><code class=\"language-go\">return fmt.Errorf(\"Error opening: %w\", err)<\/code><\/pre>\n\n\n\n<ul class=\"has-medium-font-size wp-block-list\"><li>The . assert outputs<\/li><\/ul>\n\n\n\n<pre class=\"wp-block-prismatic-blocks\"><code class=\"language-go\">Using Assert: Error not of type path error<\/code><\/pre>\n\n\n\n<ul class=\"has-medium-font-size wp-block-list\"><li>While As function outputs<\/li><\/ul>\n\n\n\n<pre class=\"wp-block-prismatic-blocks\"><code class=\"language-go\">Using As function: Error e is of type path error. Error: open non-existing.txt: no such file or directory<\/code><\/pre>\n\n\n\n<p>This is because the error returned by the <strong>openFile <\/strong>function wraps <strong>*os.Patherror<\/strong> error which is not catched by the dot(&#8216;.&#8217;) assert but is catched by <strong>As<\/strong> function<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Table of Contents OverviewCode Overview There are two ways of getting the underlying type Using the .({type}) assert If the assert succeeds then it will return the corresponding error otherwise it will&#8230;<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"spay_email":"","footnotes":"","jetpack_publicize_message":"","jetpack_is_tweetstorm":false},"categories":[1],"tags":[3,4],"class_list":["post-3923","post","type-post","status-publish","format-standard","hentry","category-tech","tag-go","tag-golang"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v22.7 - 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