Snakemake complains it can't find the file it is supposed to create in a run instruction

Question

Consider the following simple snakefile, which is an attempt to write a file in a run instruction:

rule all:
    input:
        "test.txt"

rule make_test:
    output:
        filename = "test.txt"
    run:
        with open(output.filename) as f:
            f.write("test")

Running it results in the following:

Provided cores: 1
Rules claiming more threads will be scaled down.
Job counts:
    count   jobs
    1   all
    1   make_test
    2
rule make_test:
    output: test.txt
Error in job make_test while creating output file test.txt.
RuleException:
FileNotFoundError in line 10 of /tmp/Snakefile:
[Errno 2] No such file or directory: 'test.txt'
  File "/tmp/Snakefile", line 10, in __rule_make_test
Will exit after finishing currently running jobs.
Exiting because a job execution failed. Look above for error message

I'm surprised of this FileNotFoundError . Obviously, I didn't find the correct way to tell snakemake that this is the file I want the rule make_test to create.

I also tried the following modification of the output syntax:

rule all:
    input:
        "test.txt"

rule make_test:
    output:
        "test.txt"
    run:
        with open(output[0]) as f:
            f.write("test")

The error is the same.

What's happening?

Answer 1

I found the reason for the bug: I had simply forgotten to open the file in write mode:

rule all: input: "test.txt"

The following works:

rule make_test:
    output:
        "test.txt"
    run:
        with open(output[0], "w") as f:
            f.write("test")