It is always said that Python is not so efficient as other languages such as C/C++, Java etc. And it is also recommended to write the bottleneck part in C. But I've never run into such problems, maybe it's because most of the time it is the way you solve the problem rather than the efficiency of the language to bother. 
Can anybody illustrate any real circumstances? Some simple codes will be great.
There isn't a specific set of circumstances in which C or C++ win. Pretty much any CPU-heavy code you write in C or C++ will run many times faster than the equivalent Python code.
If you haven't noticed, it's simply because, for the problems you've had to solve in Python, performance has never been an issue.
Any real examples to show python's inefficiency？