Give an example of 25 hidden features in Python _python

Note: The familiar tips for generators, adorners, and exchange variables are ignored here
1. function parameter Unpack

It's a cliché:

def foo (x, y):
  print x, y
 
alist = [1, 2]
adict = {' x ': 1, ' Y ': 2}
 
foo (*alist) # 1, 2
foo (**adict) # 1, 2

2. Chain comparison operator

>>> x = 3
>>> 1 < x < 5
true
>>> 4 > x >=3
True

3. Note The default parameters of the function

>>> def foo (x=[]):
...   X.append (1)
...   Print x ...
>>> foo ()
[1]
>>> foo ()
[1, 1]

A safer approach:

>>> def foo (x=none):
...   If X is None:
....     x = [] ...   X.append (1)
...   Print x ...
>>> foo ()
[1]
>>> foo ()
[1]
>>>

4. The dictionary has a Get () method

Dct.get (Key[,default_value]), when a key is not found in the dictionary DCT, get returns Default_value

Sum[value] = sum.get (value, 0) + 1

5. Formatted with keywords

>>> print "Hello% (name) s!"% {' name ': ' James '}
hello James!
>>> print "I am years% (age) I years old"% {' age ': @
I am years years old

To update some of the formatting:

>>> print "Hello {name}!". Format (name= "James")
Hello James!

Quick some of the template engine flavor:
6. For...else Grammar

>>> for I in (1, 3, 5):
...   If I% 2 = 0:
...     Break
... else:
...   Print "Var i is always an odd"
...
var i is always an odd
>>>

Else statement blocks are executed after the end of the loop, unless the break is executed in a loop block
7. Special methods of Dict __missing__

That was introduced after Python 2.5. This method is executed when the key is not found.

>>> class Dict (Dict):
...  def __missing__ (self, key):
...   Self[key] = []
...   return Self[key]
...
>>> DCT = Dict ()
>>> dct["foo"].append (1)
>>> dct["foo"].append (2)
> >> dct["foo"]
[1, 2]

It's like Collections.defaultdict, isn't it?

>>> from collections import defaultdict
>>> DCT = defaultdict (list)
>>> dct["foo" ]
[]
>>> dct["Bar"].append ("Hello")
>>> DCT
defaultdict (<type ' list ' > , {' foo ': [], ' Bar ': [' Hello ']}]

8. Step parameters for slicing operations

You can also use the step size-to reverse the list:

9. Another kind of string connection

>>> name = "Wang" "Wanghong"
>>> name
' "

Connect Multiple lines:

>>> Name = "Wang" \
... "" "
>>> Name
' wanghong '
10. "_"
 
>>> Range (4)
[0, 1, 2, 3]
>>> _
[0, 1, 2, 3] in the Python interpreter

_ that is the last value returned by the Python interpreter
Python Descriptor

The Python descriptor is a magical thing in Python, and methods are all descriptors. No more examples
Zen

Import this

13. Nested list derivation

>>> [(I, J) for I in range (3) for J in Range (i)]
[(1, 0), (2, 0), (2, 1)]
try/except/else
 
Try:
 put_4000000000_volts_through_it (Parrot)
except Voom:
 print "' E ' pining!"
else:
 print "This parrot is no more!"
Finally:
 End_sketch ()

Print redirect output to file

>>> Print >> open ("Somefile", "w+"), "Hello World"

Note Open mode: "w+" and not "w", of course "a" is OK
16. Ellipsis

In Python3 you can use the ellipsis directly in this grammar:

Python 3.2 (r32:88445, OCT, 14:09:50)
[GCC 4.5.2] on linux2
Type ' help ', ' copyright ', ' credits ' or ' lice NSE "For more information.
>>> ... Ellipsis

What about the Python2?

>>> class C (object):
... def __getitem__ (self, item):
...  Return Item
...
>>> C () [1:2, ..., 3]
(Slice (1, 2, None), ellipsis, 3)
>>>

Tuple unpack in Python3

I really wish Python2 would have done the same thing:

>>> A, B, *rest = Range (Ten)
>>> a
0
>>> b
1
>>> Rest
[2, 3, 4, 5, 6, 7, 8, 9]
>>>

Of course, you can also take out the last one:

>>> A, second, *rest, last = range
>>>
0 (
>>> second
1)
>>> last
9
>>> rest
[2, 3, 4, 5, 6, 7, 8]

Pow () and a third parameter

We all know that the built-in function Pow,pow (x,y) is X**y

But it can also have a third parameter:

>>> POW (4, 2, 2)
0
>>> Pow (4, 2, 3)
1

In fact, the third parameter is to seek modulo: pow (x,y,z)? (x**y)?%z

Note that the built-in POW and math.pow are not a function, which accepts only 2 parameters
Enumerate also has a second parameter.

Enumerate is awesome, you can give us index and sequence values, but it has a second argument:

>>> LST = ["A", "B", "C"]
>>> list (Enumerate (LST, 1))
[(1, ' a '), (2, ' B '), (3, ' C ')]

This parameter is used to: indicate the starting value of the index
20. Explicit declaration of a set

To create a new collection, we will:

>>> set ([1,2,3])

After Python 2.7, you can write this:

>>> {1,2,3}
set ([1, 2, 3])

21. Use a slice to delete a segment of a sequence

>>> a = [1, 2, 3, 4, 5, 6, 7]
>>> a[1:4] = []
>>> a
[1, 5, 6, 7]

Of course, with Dela[1:4] is also possible

Get rid of even-numbered items (even index):

>>> a = [0, 1, 2, 3, 4, 5, 6, 7]
>>> del A[::2]
>>> a
[1, 3, 5, 7]

Isinstance can receive a tuple

This is really little known, we can use Isinstance (x, (Float,int)) to determine whether X is the number:

>>> isinstance (1, (float, int))
true
>>> isinstance (1.3, (float, int))
true
>>> isinstance ("1.3", (float, int))
False

So for the third Test, you can see what's going on by adding STR to the tuple:

>>> isinstance ("1.3", (float, int, str))
True

That is, the relationship within that tuple, as long as one of the instances returns true
23. Infinite recursion in a dictionary

>>> A, B = {}, {}
>>> a[' b '] = b
>>> b[' a '] = a
>>> a
{' B ': {' a ': {.. .}}}

Of course you can create an infinite loop in a list:

>>> A, B = [], []
>>> a.append (b)
>>> b.append (a)
>>> a
[[[...]]]

I really don't know what it's for, but it's fun, isn't it?
Python can recognize numbers in Unicode

So Python is awesome:

>>> int (u ' 1234 ')
1234

Not only are ASCII strings recognizable, even Unicode can be.
25. Properties that cannot be accessed

The person who answered the answer is too bad:

 
>>> class O (object):p
...
>>> o = O ()
>>> setattr (o, "can ' t touch this", 123)
>>> O.can ' t touch this
 File &L T;stdin> ", line 1
  o.can ' t touch this
           ^
syntaxerror:eol while scanning string literal
>>>< C10/>file "<stdin>", line 1
  o.can ' t touch this
           ^
syntaxerror:eol while scanning string literal

However, you can use the SetAttr to set the properties and use the GetAttr to remove