instead it should check for linux once, store that linux is used and not even once check for dylib

strace extract

[pid 23025] stat64("//lib/libreadline.dylib", 0xbfd27c1c) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("//lib/libreadline.so", 0xbfd27c1c) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("//lib/libreadline.a", 0xbfd27c1c) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("/lib64/libreadline.dylib", 0xbfd27c1c) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("/lib64/libreadline.so", 0xbfd27c1c) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("/lib64/libreadline.a", 0xbfd27c1c) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("/usr/lib64/libreadline.dylib", 0xbfd27c1c) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("/usr/lib64/libreadline.so", 0xbfd27c1c) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("/usr/lib64/libreadline.a", 0xbfd27c1c) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("/lib/libreadline.dylib", 0xbfd27c1c) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("/lib/libreadline.so", 0xbfd27c1c) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("/lib/libreadline.a", 0xbfd27c1c) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("/usr/lib/libreadline.dylib", 0xbfd27c1c) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("/usr/lib/libreadline.so", 0xbfd27c1c) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("/usr/lib/libreadline.a", 0xbfd27c1c) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("//lib/libncursesw.dylib", 0xbfd27c1c) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("//lib/libncursesw.so", 0xbfd27c1c) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("//lib/libncursesw.a", 0xbfd27c1c) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("/lib64/libncursesw.dylib", 0xbfd27c1c) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("/lib64/libncursesw.so", 0xbfd27c1c) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("/lib64/libncursesw.a", 0xbfd27c1c) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("/usr/lib64/libncursesw.dylib", 0xbfd27c1c) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("/usr/lib64/libncursesw.so", 0xbfd27c1c) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("/usr/lib64/libncursesw.a", 0xbfd27c1c) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("/lib/libncursesw.dylib", 0xbfd27c1c) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("/lib/libncursesw.so", 0xbfd27c1c) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("/lib/libncursesw.a", 0xbfd27c1c) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("/usr/lib/libncursesw.dylib", 0xbfd27c1c) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("/usr/lib/libncursesw.so", 0xbfd27c1c) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("/usr/lib/libncursesw.a", 0xbfd27c1c) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("//lib/libncurses.dylib", 0xbfd27c1c) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("//lib/libncurses.so", 0xbfd27c1c) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("//lib/libncurses.a", {st_mode=S_IFREG|0644, st_size=431218, ...}) = 0
[pid 23025] stat64("//lib/libcrypt.dylib", 0xbfd27c1c) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("//lib/libcrypt.so", 0xbfd27c1c) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("//lib/libcrypt.a", {st_mode=S_IFREG|0644, st_size=8, ...}) = 0
[pid 23025] stat64("//include/openssl/ssl.h", {st_mode=S_IFREG|0644, st_size=93660, ...}) = 0
[pid 23025] stat64("//include/krb5.h", 0xbfd27cac) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("./krb5.h", 0xbfd27cac) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("Include/krb5.h", 0xbfd27cac) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("./Include/krb5.h", 0xbfd27cac) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("/src/Python-2.7.2/Include/krb5.h", 0xbfd27cac) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("/src/Python-2.7.2/krb5.h", 0xbfd27cac) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("/usr/include/krb5.h", 0xbfd27cac) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("/usr/kerberos/include/krb5.h", 0xbfd27cac) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("//lib/libssl.dylib", 0xbfd27b3c) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("//lib/libssl.so", 0xbfd27b3c) = -1 ENOENT (No such file or directory)
[pid 23025] stat64("//lib/libssl.a", {st_mode=S_IFREG|0644, st_size=448496, ...}) = 0

The list of file extensions is written in _PyImport_DynLoadFiletab constant. This list depends on the OS. For Linux, the list is defined in: Python/dynload_shlib.c.

I don't understand because in Python 2.7, Python/dynload_shlib.c doesn't contain ".dylib".

I know this is an old-ish issue, but I can't reproduce anything like this on Debian Wheezy with either Python 2.7 or tip (3.x). I think we need more details of what you're trying to do here Roger.

1. What exactly did you do to reproduce the strace output below?
2. What flavor of Linux did you attempt this on?

Further, a more detailed description of what you expected to happen vs. what actually happened would be a big help here.

As Victor says, .dylib shouldn't appear in strace output on Linux at all (and my testing seems to confirm this).

i'll try to reproduce this later.

looking at the below strace output, python's behaviour seems pretty stupid tho,

for example if /lib is the only existing path out of /lib, /lib64, /usr/lib/, /usr/local/lib, etc etc, it should check for all these directories once, and then not try to open .so in unexisting directories over and over again, inflicting multiple syscalls for each dso.

Which was the command run under strace exactly?

Since this has been pending for over two months I'm claiming it's outdated.