writes every `skip` line number

decimate.py

infile = open("input.txt")
outfile = open("output.txt", "w")

count = 0
skip = 30
for line in infile:
    if count >= skip:
        outfile.write(line)
        count = 0
    else:
        count += 1

input.txt

record 0
record 1
record 2
record 3
record 4
record 5
record 6
record 7
record 8
record 9
record 10
record 11
record 12
record 13
record 14
record 15
record 16
record 17
record 18
record 19
record 20
record 21
record 22
record 23
record 24
record 25
record 26
record 27
record 28
record 29
record 30
record 31
record 32
record 33
record 34
record 35
record 36
record 37
record 38
record 39
record 40
record 41
record 42
record 43
record 44
record 45
record 46
record 47
record 48
record 49
record 50
record 51
record 52
record 53
record 54
record 55
record 56
record 57
record 58
record 59
record 60
record 61
record 62
record 63
record 64
record 65
record 66
record 67
record 68
record 69
record 70
record 71
record 72
record 73
record 74
record 75
record 76
record 77
record 78
record 79
record 80
record 81
record 82
record 83
record 84
record 85
record 86
record 87
record 88
record 89
record 90
record 91
record 92
record 93
record 94
record 95
record 96
record 97
record 98
record 99
record 100
record 101
record 102
record 103
record 104
record 105
record 106
record 107
record 108
record 109
record 110
record 111
record 112
record 113
record 114
record 115
record 116
record 117
record 118
record 119

output.txt

record 30
record 61
record 92