unexpected behavior in python

wat.py

myhome$ ipython
Python 3.7.2 (default, Feb 12 2019, 08:15:36) 
Type 'copyright', 'credits' or 'license' for more information
IPython 6.5.0 -- An enhanced Interactive Python. Type '?' for help.

In [1]: # comparisons

In [2]: a = 256

In [3]: b = 256

In [4]: a is b
Out[4]: True

In [5]: a == b
Out[5]: True

In [6]: a = 257

In [7]: b = 257

In [8]: a is b
Out[8]: False

In [9]: a == b
Out[9]: True

In [10]: id(a)
Out[10]: 4399267056

In [11]: id(b)
Out[11]: 4399267664

In [12]: a is 257
Out[12]: False

In [13]: a == 257
Out[13]: True

In [14]: a = 3

In [15]: b = 3

In [16]: a is b
Out[16]: True

In [17]: clear


In [18]: # scope

In [19]: a = 1

In [20]: def f(x):
    ...:     return a
    ...: 
    ...: 

In [21]: f(2)
Out[21]: 1

In [22]: def f(x):
    ...:     a += 1
    ...:     return a
    ...: 
    ...: 

In [23]: f(2)
---------------------------------------------------------------------------
UnboundLocalError                         Traceback (most recent call last)
<ipython-input-23-c510dc86724b> in <module>()
----> 1 f(2)

<ipython-input-22-c9c3272a51e0> in f(x)
      1 def f(x):
----> 2     a += 1
      3     return a

UnboundLocalError: local variable 'a' referenced before assignment

In [24]: def f(x):
    ...:     a = 1
    ...:     return
    ...: 
    ...: 

In [25]: a = 2

In [26]: f(3)

In [27]: a
Out[27]: 2

In [28]: def f(x):
    ...:     a = 1
    ...:     print(a)
    ...:     return
    ...: 
    ...: 

In [29]: f(3)
1

In [30]: a
Out[30]: 2

In [31]: clear


In [32]: # comparisons

In [33]: 3 is 1 + 2
Out[33]: True

In [34]: 0.3 is 0.1 + 0.2
Out[34]: False

In [35]: 0.3 == 0.1 + 0.2
Out[35]: False

In [36]: 0 is False
Out[36]: False

In [37]: 0 == False
Out[37]: True

In [38]: 0.1+0.2
Out[38]: 0.30000000000000004

In [39]: round(0.1, 1)
Out[39]: 0.1

In [40]: '' is ''
Out[40]: True

In [41]: 0 is 0
Out[41]: True

In [42]: [] is []
Out[42]: False

In [43]: {} is {}
Out[43]: False

In [44]: clear


In [45]: # comparisons

In [46]: x = (1 << 53) + 1

In [47]: x
Out[47]: 9007199254740993

In [48]: x + 1. < x
Out[48]: True

In [49]: x + 1 < x
Out[49]: False

In [50]: False is False
Out[50]: True

In [51]: True in [True]
Out[51]: True

In [52]: False is False in [True]
Out[52]: False

In [53]: (False is False) in [True]
Out[53]: True

In [54]: 0 is 0 in [1]
Out[54]: False

In [55]: (0 is 0) in [1]
Out[55]: True

In [56]: clear


In [57]: # Immutability

In [58]: a = ([1], )

In [59]: a
Out[59]: ([1],)

In [60]: a[0] += [2]
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-60-94399e3f4fce> in <module>()
----> 1 a[0] += [2]

TypeError: 'tuple' object does not support item assignment

In [61]: a
Out[61]: ([1, 2],)

In [62]: a[0].extend(3)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-62-be2fd989379d> in <module>()
----> 1 a[0].extend(3)

TypeError: 'int' object is not iterable

In [63]: a[0].extend([3])

In [64]: a
Out[64]: ([1, 2, 3],)

In [65]: [1] + 2
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-65-54324b7054ad> in <module>()
----> 1 [1] + 2

TypeError: can only concatenate list (not "int") to list

In [66]: type(a[0])
Out[66]: list

In [67]: type(a)
Out[67]: tuple

In [68]: a[0].append(3)

In [69]: a
Out[69]: ([1, 2, 3, 3],)

In [70]: clear


In [71]: # copying

In [72]: a = [1] * 3

In [73]: a
Out[73]: [1, 1, 1]

In [74]: b = a

In [75]: a[0] = 1000

In [76]: a
Out[76]: [1000, 1, 1]

In [77]: b
Out[77]: [1000, 1, 1]

In [78]: c = a[:]

In [79]: a
Out[79]: [1000, 1, 1]

In [80]: c
Out[80]: [1000, 1, 1]

In [81]: a[0] = 1

In [82]: a
Out[82]: [1, 1, 1]

In [83]: c
Out[83]: [1000, 1, 1]

In [84]: d = a.copy()

In [85]: a = [[1, 2, 3], [4, 5, 6]]

In [86]: b = a[:]

In [87]: a[0][0] = 1000

In [88]: a
Out[88]: [[1000, 2, 3], [4, 5, 6]]

In [89]: b
Out[89]: [[1000, 2, 3], [4, 5, 6]]

In [90]: import copy

In [91]: d = copy.deepcopy(a)

In [92]: a
Out[92]: [[1000, 2, 3], [4, 5, 6]]

In [93]: d
Out[93]: [[1000, 2, 3], [4, 5, 6]]

In [94]: a[0][0] = 1

In [95]: a
Out[95]: [[1, 2, 3], [4, 5, 6]]

In [96]: d
Out[96]: [[1000, 2, 3], [4, 5, 6]]

In [97]: copy.copy
Out[97]: <function copy.copy(x)>