Read multiple files line by line in Python evaluated as lazy as possible

python_lazy.py

import itertools
import glob

print(list(itertools.chain([1,2,3],[4,5,6],[7,'a'])))

print("-"*80)

### LAZY

# Writing files for test
for i in range(1,4):
    with open(f'tmp/test{i}.txt','w') as out:
        for x in range(1,10):
            print(f"File: {out.name}, Line: {x} {str(x*i)*x*i}",file=out)
        out.write("---\n")
        
    
# round parennthesis: generator
# iglob: iterable
file_handles = (open(x) for x in glob.iglob("tmp/*.txt"))

# iterate over list of filehandles:
for line in itertools.chain.from_iterable(file_handles):
    print(f"Length of line: {len(line):>4}  |  {line}",end='')

python_lazy_output.txt

[1, 2, 3, 4, 5, 6, 7, 'a']
--------------------------------------------------------------------------------
Length of line:   31  |  File: tmp/test1.txt, Line: 1 1
Length of line:   32  |  File: tmp/test1.txt, Line: 2 22
Length of line:   33  |  File: tmp/test1.txt, Line: 3 333
Length of line:   34  |  File: tmp/test1.txt, Line: 4 4444
Length of line:   35  |  File: tmp/test1.txt, Line: 5 55555
Length of line:   36  |  File: tmp/test1.txt, Line: 6 666666
Length of line:   37  |  File: tmp/test1.txt, Line: 7 7777777
Length of line:   38  |  File: tmp/test1.txt, Line: 8 88888888
Length of line:   39  |  File: tmp/test1.txt, Line: 9 999999999
Length of line:    4  |  ---
Length of line:   32  |  File: tmp/test2.txt, Line: 1 22
Length of line:   34  |  File: tmp/test2.txt, Line: 2 4444
Length of line:   36  |  File: tmp/test2.txt, Line: 3 666666
Length of line:   38  |  File: tmp/test2.txt, Line: 4 88888888
Length of line:   50  |  File: tmp/test2.txt, Line: 5 10101010101010101010
Length of line:   54  |  File: tmp/test2.txt, Line: 6 121212121212121212121212
Length of line:   58  |  File: tmp/test2.txt, Line: 7 1414141414141414141414141414
Length of line:   62  |  File: tmp/test2.txt, Line: 8 16161616161616161616161616161616
Length of line:   66  |  File: tmp/test2.txt, Line: 9 181818181818181818181818181818181818
Length of line:    4  |  ---
Length of line:   33  |  File: tmp/test3.txt, Line: 1 333
Length of line:   36  |  File: tmp/test3.txt, Line: 2 666666
Length of line:   39  |  File: tmp/test3.txt, Line: 3 999999999
Length of line:   54  |  File: tmp/test3.txt, Line: 4 121212121212121212121212
Length of line:   60  |  File: tmp/test3.txt, Line: 5 151515151515151515151515151515
Length of line:   66  |  File: tmp/test3.txt, Line: 6 181818181818181818181818181818181818
Length of line:   72  |  File: tmp/test3.txt, Line: 7 212121212121212121212121212121212121212121
Length of line:   78  |  File: tmp/test3.txt, Line: 8 242424242424242424242424242424242424242424242424
Length of line:   84  |  File: tmp/test3.txt, Line: 9 272727272727272727272727272727272727272727272727272727
Length of line:    4  |  ---