Python: string types in 2.x and 3.x

• In Python 2.x, basestring is the base class for both str and unicode, while types.StringType is str. If you want to check if something is a string, use basestring. If you want to check if something is a ascii (bytestring), use str and forget about types.
• Since Python 3.x, types not longer has StringType; str is always unicode. basestring no longer exists.

python: return inside with block

if returns value inside with statement, will the file be closed eventually?
The answer is YES

with open('myfile.txt') as f:
    return [line for line in f if len(line)>80]

python: iterables and iterators

A class with __iter__() method or __getitem__() method is iterable. The __iter__() method returns an iterator. A iterator object has a next() method (or __next__() method in python 3.0).

class Iterator(object):
    def next(self):
        pass

class Iterable(object):
    def __iter__(self):
        return Iterator()

see also

• Build a Basic Python Iterator
• What makes something iterable in python
• Understanding Python's iterator, iterable, and iteration protocols — what exactly are they?

python: len() function and __len__() method

len is a function to get the length of a collection. It works by calling an object's __len__ method.

class AClass(object):
    def __init__(self):
        self._list = [1,2,3]

    def __len__(self):
        return len(self._list)

a=AClass()
print(a.__len__()) # 3
print(len(a))      # 3

python: call another static method

class AClass(object):
    def __init__(self):
        pass

    @staticmethod
    def a():
        AClass.b()

    @staticmethod
    def b():
        pass

see also

• Python static methods - how to call a method from another method

Python: int to hex string

print(hex(255))

Result:

'0xff'

hex(255)[2:]

Result:

'ff'

Python: generates crc32 and adler32 checksum for big files

import zlib
import sys
import urllib2

def __zlib_csum(url, func):
    if isinstance(url, basestring if sys.version_info[0] < 3 else str):
        url = urllib2.Request(url)
    f = urllib2.urlopen(url)
    csum = None
    try:
        chunk = f.read(1024)
        if len(chunk)>0:
            csum = func(chunk)
            while True:
                chunk = f.read(1024)
                if len(chunk)>0:
                    csum = func(chunk, csum)
                else:
                    break
    finally:
        f.close()
    if csum is not None:
        csum = csum & 0xffffffff
    return csum
    

def crc32(url):
    return __zlib_csum(url, zlib.crc32)

def adler32(url):
    return __zlib_csum(url, zlib.adler32)

if __name__ == '__main__':
    print(hex(crc32('file:/tmp/111.zip')))
    print(hex(adler32('file:/tmp/111.zip')))