Python: check if file or directory exists

• os.path.exists to check if file or directory exists.

  import os
  os.path.exists('/tmp/1.txt')
  os.path.exists('/tmp')
  

• os.path.isfile to check if file exists.

  import os
  os.path.isfile('/tmp/1.txt')
  

Python: Get file absolute path

Use os.path.abspath function:

import os
os.path.abspath('abc/abc.txt')

Python: get the index of a char in a string

find and index method are available. The difference is that find returns -1 when what you're looking for isn't found, index throws an exception.

s='abc'
print(s.index('b')) # 1
print(s.find('b'))  # 1
print(s.find('d'))  # -1
print(s.index('d')) # exception

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