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Asked: 2026-05-28T19:46:40+00:00

What precision does numpy.float128 map to internally? Is it __float128 or long double? Or

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What precision does numpy.float128 map to internally? Is it __float128 or long double? Or something else entirely?

A potential follow on question if anybody knows: is it safe in C to cast a __float128 to a (16 byte) long double, with just a loss in precision? (this is for interfacing with a C lib that operates on long doubles).

Edit: In response to the comment, the platform is ‘Linux-3.0.0-14-generic-x86_64-with-Ubuntu-11.10-oneiric’. Now, if numpy.float128 has varying precision dependent on the platform, that is also useful knowledge for me!

Just to be clear, it is the precision I am interested in, not the size of an element.

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  1. Editorial Team

    It’s quite recommended to use longdouble instead of float128, since it’s quite a mess, ATM. Python will cast it to float64 during initialization.

    Inside numpy, it can be a double or a long double. It’s defined in npy_common.h and depends of your platform. I don’t know if you can include it out-of-the-box into your source code.

    If you don’t need performance in this part of your algorithm, a safer way could be to export it to a string and use strold afterwards.

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