↓実験
>>> for i in range(10): print open("file.txt")
<open file 'file.txt', mode 'r' at 0x00A9A338>
<open file 'file.txt', mode 'r' at 0x00A9A338>
<open file 'file.txt', mode 'r' at 0x00A9A338>
<open file 'file.txt', mode 'r' at 0x00A9A338>
<open file 'file.txt', mode 'r' at 0x00A9A338>
<open file 'file.txt', mode 'r' at 0x00A9A338>
<open file 'file.txt', mode 'r' at 0x00A9A338>
<open file 'file.txt', mode 'r' at 0x00A9A338>
<open file 'file.txt', mode 'r' at 0x00A9A338>
<open file 'file.txt', mode 'r' at 0x00A9A338>

仕様のどこ見ればいい?