gocolly/colly · error

ErrMissingURL

ErrMissingURL

Error message

Missing URL

What it means

ErrMissingURL is returned when Visit() is called with an empty URL string. Colly cannot construct a request without a target URL, so it fails fast before any network I/O. It guards against programming mistakes such as an unset variable or a failed string extraction.

Source

Thrown at colly.go:225

// The key type is unexported to prevent collisions with context keys defined in
// other packages.
type key int

// ProxyURLKey is the context key for the request proxy address.
const (
	ProxyURLKey key = iota
	CheckRevisitKey
)

// The prefix for environment variables of Colly settings
const envVariablePrefix = "COLLY_"

var (
	// ErrForbiddenDomain is the error thrown if visiting
	// a domain which is not allowed in AllowedDomains
	ErrForbiddenDomain = errors.New("Forbidden domain")
	// ErrMissingURL is the error type for missing URL errors
	ErrMissingURL = errors.New("Missing URL")
	// ErrMaxDepth is the error type for exceeding max depth
	ErrMaxDepth = errors.New("Max depth limit reached")
	// ErrForbiddenURL is the error thrown if visiting
	// a URL which is not allowed by URLFilters
	ErrForbiddenURL = errors.New("ForbiddenURL")

	// ErrNoURLFiltersMatch is the error thrown if visiting
	// a URL which is not allowed by URLFilters
	ErrNoURLFiltersMatch = errors.New("No URLFilters match")
	// ErrRobotsTxtBlocked is the error type for robots.txt errors
	ErrRobotsTxtBlocked = errors.New("URL blocked by robots.txt")
	// ErrNoCookieJar is the error type for missing cookie jar
	ErrNoCookieJar = errors.New("Cookie jar is not available")
	// ErrNoPattern is the error type for LimitRules without patterns
	ErrNoPattern = errors.New("No pattern defined in LimitRule")
	// ErrEmptyProxyURL is the error type for empty Proxy URL list
	ErrEmptyProxyURL = errors.New("Proxy URL list is empty")
	// ErrAbortedAfterHeaders is the error returned when OnResponseHeaders aborts the transfer.

View on GitHub (pinned to 17d1d6ca92)

Solutions

1. Check the argument before calling Visit: only call it when len(u) > 0
2. Log the URL value at the call site to find where the empty string originates
3. For collected links, verify the attribute exists (e.g. e.Attr("href") != "") before queueing
4. If the URL comes from config/env, validate it at startup

Example fix

// before
u := e.Attr("href")
c.Visit(u) // panics into ErrMissingURL when href=""
// after
u := e.Attr("href")
if u != "" {
    c.Visit(u)
}

if target == "" {
    return fmt.Errorf("no URL to visit")
}
u, err := url.Parse(target)
if err != nil || u.Scheme == "" || u.Host == "" {
    return fmt.Errorf("invalid URL %q", target)
}

func isNonEmptyURL(s string) bool {
    u, err := url.Parse(s)
    return err == nil && s != "" && u.Host != ""
}

if err := c.Visit(target); err != nil {
    if errors.Is(err, colly.ErrMissingURL) {
        log.Printf("empty URL at %s, skipping", callSite)
        return nil
    }
    return err
}

When it happens

Trigger: Calling c.Visit("") or c.Visit(u) where u is the empty string; passing the result of an extraction/attribute lookup that came back empty (e.g. href="" or a missing query parameter); building the URL from optional config that was not provided.

Common situations: Scraping loops where a link attribute is missing on some pages so href is ""; command-line/config-driven crawlers where the --url flag or env var was never set; concatenating URL parts and ending up with an empty string.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• ErrForbiddenDomain
• ErrMaxDepth
• ErrForbiddenURL
• ErrNoURLFiltersMatch
• ErrRobotsTxtBlocked

AI-assisted analysis of gocolly/colly@17d1d6ca92 (2026-08-30). Data as JSON: /api/errors/e3ec048d4eb611ad. Report an issue: GitHub.