To begin with let’s just have look at the old school of lambda in Python.

As we know, lambda x: return x**2 is exactly equivalent to

def squared(x): return x**2

Now look at this

>>> f1 = lambda x,y,z: x+1, y+1, z+1
>>> print(f1(1,1,1))

What will you get on the screen? A tuple of (2, 2, 2)? No. You get an error instead.

Traceback (most recent call last):
  File "<pyshell#7>", line 1, in <module>
    print(f1(1,1,1))
TypeError: 'tuple' object is not callable</module>

Now that the “returning part” of lambda is covering the contents before comma only, let’s explicitly add the brackets:

As noted in Python 3 documention, behaviors of +, -, *, etc. can be redefined.

Specially, I would like to take notes on some special yet common methods.

__repr__ method

This method is called when you apply print on the instance. Could be useful when debugging class related problems.

__enter__, __exit__ methods

Those two methods are key components of a context manager. Refer to https://jeffknupp.com/blog/2016/03/07/python-with-context-managers/ to get a sense on how to manage the fragile resouces with context manager.

简而言之，如果你通过Windows自带的命令提示符（cmd）启动了pip，并且看到了类似下面的错误提示
in console_to_strreturn s.decode(sys.__stdout__.encoding)UnicodeDecodeError: 'utf-8' codec can't decode byte 0xb6 in position 33: invalid start byte
多半是因为命令提示符当前使用的代码页不是UTF-8。使用命令chcp，将当前代码页切换成UTF-8即可解决：