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Python3.x Python List Common Operations

Python3 Examples

Examples

>>> li =["a","b","mpilgrim","z","example"]

>>> li

['a','b','mpilgrim','z','example']

>>> li

'b'

2. List Negative Indexing

Examples

>>> li 

['a','b','mpilgrim','z','example']

>>> li

'example'

>>> li

'mpilgrim'

>>> li 

['a','b','mpilgrim','z','example']

>>> li[1:3]

['b','mpilgrim']

>>> li[1:-1]

['b','mpilgrim','z']

>>> li[0:3]

['a','b','mpilgrim']

3. Adding Elements to a List

Examples

>>> li 

['a','b','mpilgrim','z','example']

>>> li.append("new")

>>> li 

['a','b','mpilgrim','z','example','new']

>>> li.insert(2,"new")

>>> li 

['a','b','new','mpilgrim','z','example','new']

>>> li.extend(["two","elements"])

>>> li 

['a','b','new','mpilgrim','z','example','new','two','elements']

4. Searching a List

Examples

>>> li 

['a','b','new','mpilgrim','z','example','new','two','elements']

>>> li.index("example")

5

>>> li.index("new")

2

>>> li.index("c")

 Traceback (innermost last):

File "", line 1,in ?

ValueError: list.index(x): x not in list

>>>"c"in li

False

5. Removing Elements from a List

Examples

>>> li 

['a','b','new','mpilgrim','z','example','new','two','elements']

>>> li.remove("z")

>>> li 

['a','b','new','mpilgrim','example','new','two','elements']

>>> li.remove("new")# Removes the first occurrence of a value

>>> li 

['a','b','mpilgrim','example','new','two','elements']# The second 'new' is not removed

>>> li.remove("c")# If the value is not found in the list, Python raises an exception

 Traceback (innermost last): 

File "", line 1,in ? 

ValueError: list.remove(x): x not in list

>>> li.pop()# pop does two things: removes the last element of the list, then returns the removed element's value.

'elements'

>>> li 

['a','b','mpilgrim','example','new','two']

6. List Operators

Examples

>>> li =['a','b','mpilgrim']

>>> li = li + ['example','new']

>>> li 

['a','b','mpilgrim','example','new']

>>> li +=['two']

>>> li 

['a','b','mpilgrim','example','new','two']

>>> li =[1,2] * 3

>>> li 

[1,2,1,2,1,2]

7. Using join to Concatenate a List into a String

Examples

>>> params ={"server":"mpilgrim","database":"master","uid":"sa","pwd":"secret"}

>>>["%s=%s" % (k, v)for k, v in params.items()]

['server=mpilgrim','uid=sa','database=master','pwd=secret']

>>>";".join(["%s=%s" % (k, v)for k, v in params.items()])

'server=mpilgrim;uid=sa;database=master;pwd=secret'

join can only be used with lists whose elements are strings; it does not perform any type coercion. Joining a list that contains one or more non-string elements will raise an exception.

8. Splitting a String into a List

Examples

>>> li =['server=mpilgrim','uid=sa','database=master','pwd=secret']

>>> s =";".join(li)

>>> s 

'server=mpilgrim;uid=sa;database=master;pwd=secret'

>>> s.split(";")

['server=mpilgrim','uid=sa','database=master','pwd=secret']

>>> s.split(";",1)

['server=mpilgrim','uid=sa;database=master;pwd=secret']

split is the opposite of join; it splits a string into a multi-element list.

Note that the delimiter (";") is completely removed; it does not appear in any element of the returned list.

split accepts an optional second parameter, which is the number of times to split.

9. List Comprehensions

Examples

>>> li =[1,9,8,4]

>>>[elem*2 for elem in li]

[2,18,16,8]

>>> li

[1,9,8,4]

>>> li =[elem*2 for elem in li]

>>> li 

[2,18,16,8]

10. Dictionary Comprehensions

Examples

>>> params ={"server":"mpilgrim","database":"master","uid":"sa","pwd":"secret"}

>>> params.keys()

 dict_keys(['server','database','uid','pwd'])

>>> params.values()

 dict_values(['mpilgrim','master','sa','secret'])

>>> params.items()

 dict_items([('server','mpilgrim'),('database','master'),('uid','sa'),('pwd','secret')])

>>>[k for k, v in params.items()]

['server','database','uid','pwd']

>>>[v for k, v in params.items()]

['mpilgrim','master','sa','secret']

>>>["%s=%s" % (k, v)for k, v in params.items()]

['server=mpilgrim','database=master','uid=sa','pwd=secret']

11. Filtering a List

Examples

>>> li =["a","mpilgrim","foo","b","c","b","d","d"]

>>>[elem for elem in li if len(elem)>1]

['mpilgrim','foo']

>>>[elem for elem in li if elem !="b"]

['a','mpilgrim','foo','c','d','d']

>>>[elem for elem in li if li.count(elem)==1]

['a','mpilgrim','foo','c']

Python3 Examples