每天CookBook之Python-105

• 读取嵌套和可变长的二进制数据

import struct
import itertools

polys = [
    [(1.0, 2.5), (3.5, 4.0), (2.5, 1.5)],
    [(7.0, 1.2), (5.1, 3.0), (0.5, 7.5), (0.8, 9.0)],
    [(3.4, 6.3), (1.2, 0.5), (4.6, 9.2)],
]


def write_polys(filename, polys):

    flattened = list(itertools.chain(*polys))
    min_x = min(x for x, y in flattened)
    max_x = max(x for x, y in flattened)
    min_y = min(y for x, y in flattened)
    max_y = max(y for x, y in flattened)

    with open(filename, 'wb') as f:
        f.write(struct.pack('<iddddi',
                            0x1234,
                            min_x, min_y,
                            max_x, max_y,
                            len(polys)))

        for poly in polys:
            size = len(poly) * struct.calcsize('<dd')
            f.write(struct.pack('<i', size+4))
            for pt in poly:
                f.write(struct.pack('<dd', *pt))


def read_polys(filename):
    with open(filename, 'rb') as f:

        header = f.read(40)
        file_code, min_x, min_y, max_x, max_y, num_polys = struct.unpack('<iddddi', header)
        polys = []
        for n in range(num_polys):
            pbytes, = struct.unpack('<i', f.read(4))
            poly = []
            for m in range(pbytes // 16):
                pt =  struct.unpack('<dd', f.read(16))
                poly.append(pt)
            polys.append(poly)

        return polys

write_polys('polys.bin', polys)
print(read_polys('polys.bin'))

[[(1.0, 2.5), (3.5, 4.0), (2.5, 1.5)], [(7.0, 1.2), (5.1, 3.0), (0.5, 7.5), (0.8, 9.0)], [(3.4, 6.3), (1.2, 0.5), (4.6, 9.2)]]