#in operator

The SQL IN operator allows you to specify multiple values in WHERE Clause .it used to reduce the multiple condition OR in the SELECT, INSERT, UPDATE, or DELETE Statement. Syntax SELECT columnname(s) FROM tablename WHERE columnname IN (value1, value2, ...valueN);   Example Consider a table named “Students” and the table have some…

Python *in* operator is membership test operator. *Examples:* List ----

In [1]: python_webframeworks = ['flask', 'django', 'pylons', 'pyramid', 'brubeck'] In [2]: 'flask' in python_webframeworks Out[2]: True In [3]: 'web.py' in python_webframeworks Out[3]: False

*in* operator iterates over the list of elements and returns  *True* or *False*. *What about nested list?*

In [4]: webframeworks = [['flask', 'django', 'pyramid'],['rails', 'sintara'],['zend', 'symfony']] In [5]: 'flask' in webframeworks Out[5]: False

*in* isnt handy for nested list, unless it is overriden.  Dict ----  *in* operator against dictionary checks for the presence of key.

In [7]: person = {'name': 'kracekumar', 'country': 'India', 'os': 'Linux', 'programming_languages': {'web': 'php', 'multi_paradigm': ['python', 'ruby', 'java', 'c#']}} In [8]: 'name' in person Out[8]: True

*in* doesnt check inside key if value is dict

In [9]: 'web' in person Out[9]: False

In case if dict needs to look into keys whose value is dict, you need to override  `__contains__`. At the end of the blog post I will explain, now lets create a class which is inherited from dict and try the same experiment.

In [10]: class Person(dict):    ....:     pass    ....:  In [11]: p = Person() In [12]: p Out[12]: {} In [13]: p['name'] = 'krace' In [14]: p Out[14]: {'name': 'krace'}

To make things simple  `__init__` and other `magic methods` are omitted. 

In [15]: 'name' in p Out[15]: True

Set ---

In [16]: conferences_attended = set(['Pycon - India', 'code retreat', 'JSFOO', 'Meta Refresh'])  In [17]: 'Pycon - India' in conferences_attended Out[17]: True

Generators ----------

In [18]: 2 in xrange(3) Out[18]: True

Nested list -----------

In [19]: nested_list = [[1, 3, 5, 7], [2, 4, 6, 8], []] In [20]: [] in nested_list Out[20]: True In [21]: [2, 4, 6, 8] in nested_list Out[21]: True

In case of nested list checking whether inner list is empty is pretty handy. Strings -------

In [23]: message = " Python is simple and powerful programming language " In [24]: "Python" in message Out[24]: True In [25]: message.find("Python") Out[25]: 1 In [26]: message = "Python is simple and powerful programming language " In [27]: message.find("Python") Out[27]: 0

*in* can be used to check the presence of a sequence or substring against string. In other languages there will be function to check for substring, but in python its very straightforward. This is one of the reason why I love Python. **NOTE:**  Dont use find method in string to find the existence of a substring because it will return the position. 

if message.find(text):       #lets do what we need do else:       #fall back

Above snippet is wrong since text might be present in the beginning which will result 0 and if condition will fail. Files  -----

In [29]: with open('test.txt', 'w') as f:      f.write(" some text for checking")    ....:  In [32]: with open('test.txt', 'r') as f:      for text in f:          print 'some' in text    ....:          True

overriding `__contains__` ----  Consider a `class Person`  which is inherited from *dict* and override `__contains__`

In [153]: class Person(dict):    .....:     def __contains__(self, item):    .....:         for key in self:    .....:             if key is item:    .....:                 return True    .....:             else:    .....:                 if isinstance(self[key], dict):    .....:                     for k in self[key]:    .....:                         if k is item:    .....:                             return True    .....:         else:    .....:             return False    .....:          In [154]: p = Person() In [155]: p['skills'] = {'programming_languages': {'web': ['php'], 'multi_paradigm': ['python', 'ruby', 'c#']} } In [156]: 'skills' in p Out[156]: True In [157]: 'programming_languages' in p Out[157]: True In [158]: 'web' in p Out[158]: False