Jenkins "unable to resolve class Declarative" error when building Python script - docker

Question

I began learning to use to Jenkins and wanted to make it run a Python script of mine automatically. I followed their tutorial and created a new Project called Pipeline Test.
I've also added the GitHub repo of a Python script I wanted to test (https://github.com/mateasmario/spam-bot).
As you can see, I've created a Jenkinsfile in that repo. Because my script is called spam-bot.py, I want my Jenkinsfile to run that script every time I click "Build now" inside Jenkins. This is the content of my Jenkinsfile:
Jenkinsfile (Declarative Pipeline)
pipeline {
agent { docker { image 'python:3.10.1-alpine' } }
stages {
stage('build') {
steps {
sh 'python spam-bot.py'
}
}
}
}
The problem is, whenever I click "Build now", my build fails and the console outputs the following error:
Started by user Mario Mateas
Obtained Jenkinsfile from git https://github.com/mateasmario/spam-bot.git
org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed:
WorkflowScript: 1: unable to resolve class Declarative
# line 1, column 26.
Jenkinsfile (Declarative Pipeline)
^
1 error
at org.codehaus.groovy.control.ErrorCollector.failIfErrors(ErrorCollector.java:310)
at org.codehaus.groovy.control.CompilationUnit.applyToSourceUnits(CompilationUnit.java:958)
at org.codehaus.groovy.control.CompilationUnit.doPhaseOperation(CompilationUnit.java:605)
at org.codehaus.groovy.control.CompilationUnit.compile(CompilationUnit.java:554)
at groovy.lang.GroovyClassLoader.doParseClass(GroovyClassLoader.java:298)
at groovy.lang.GroovyClassLoader.parseClass(GroovyClassLoader.java:268)
at groovy.lang.GroovyShell.parseClass(GroovyShell.java:688)
at groovy.lang.GroovyShell.parse(GroovyShell.java:700)
at org.jenkinsci.plugins.workflow.cps.CpsGroovyShell.doParse(CpsGroovyShell.java:142)
at org.jenkinsci.plugins.workflow.cps.CpsGroovyShell.reparse(CpsGroovyShell.java:127)
at org.jenkinsci.plugins.workflow.cps.CpsFlowExecution.parseScript(CpsFlowExecution.java:571)
at org.jenkinsci.plugins.workflow.cps.CpsFlowExecution.start(CpsFlowExecution.java:523)
at org.jenkinsci.plugins.workflow.job.WorkflowRun.run(WorkflowRun.java:334)
at hudson.model.ResourceController.execute(ResourceController.java:99)
at hudson.model.Executor.run(Executor.java:432)
Finished: FAILURE
I looked up for this error on the internet, but didn't find any useful information, and that's why I decided to ask here.
I also don't have any Docker container configured. Do I need to configure one? I took a look over Jenkins' documentation for Docker, but didn't see any useful information for adding a Python image (as the one mentioned at the beginning of the Jenkinsfile) to the container.

Answer 1

Your Jenkinsfile contains invalid syntax on the first line, which is why the error is being thrown. Assuming you intended that first line to be a comment, you can modify the pipeline code to be:
// Jenkinsfile (Declarative Pipeline)
pipeline {
...
}
and your pipeline code will have valid syntax.