Nextflow: How to initialize channel from CSV file but only after CSV file created?

Process A runs a python script which generates a CSV file in some absolute file path say "/mnt/x/blah.csv".

Process B needs to consume the CSV file line by line, after process A has finished.

The trouble is that the channel to input to B is initialized at the beginning before "/mnt/x/blah.csv" is created by A so it fails. This occurs even if I collect the result of process A as input to process B (because channel is initialized regardless)

nextflow.enable.dsl=2

def get_the_csv_records() {
Channel
.fromPath("/mnt/x/blah.csv")
.map{ it.TheFieldIWant }
}

process process_a() {

output:
val("I'm done!")

'''
python.exe ./make_the_csv_file.py --path=/mnt/x/blah.csv
'''
}

process process_b() {

input:
val(previous_step_is_done_dummy_variable)
val(the_field)

'''
echo ${the_field} ''' } workflow do_a_then_b{ process_a() process_b(process_a.out.collect(), get_the_csv_records()) }  This fails immediately with "No such file: /mnt/x/blah.csv" because the channel is initialized before process_a has even started. Same behavior occurs when using concat:  process_b(process_a.out.collect().concat(get_the_csv_records()))  Question: 1. How can I make process_b() only read the input channel from the CSV file after it is created? 2. Alternatively, can I make process_b() trigger when process_a() is finished some other way? 3. Alternatively, can I add an intermediate step which reads process_a.out.collect(), then reads /mnt/x/blah.csv and outputs it into some existing channel which process_b() takes as input? 4. Another approach - what is the way to create a channel which outputs the results of splitCsv, when the file is not known at compile time? 1 Answer Assuming you are using latest version of Nextflow which support the DSL 2 language, and assuming your python process output the file blah.csv, then the easiest way to move forward will be: process process_a() { output: path("blah.csv") ''' python.exe ./make_the_csv_file.py --path=blah.csv ''' } process process_b() { input: val(the_field) ''' echo${the_field}
'''

}

workflow do_a_then_b{
process_a() \
| splitCsv \
| map{ it -> it.TheFieldIWant } \
| process_b
}


You can also refer to the nextflow google group.