Python

Don't be an "X developer". You're selling yourself short, if you define yourself by the technology you most frequently use. If you want a great career as a developer, you need to be well versed in a bunch of different technologies. The best developers I know are effective at multiple layers of the stack. They write code in high-level languages such as Python, drop down to C for performance-intensive stuff, are familiar with HTML, CSS and JavaScript, know when to pick up a new technology like Varnish or Solar, know how to set up a Linux server, know how DNS works, tinker with Objective-C on the side. There is nothing wrong with specializing in one particular programming language, but don't let it define you. The more strings you have in your bow the easier you'll find it to land your ideal job (and the more you'll be able to charge for your services).

Currently, Python is doing to Java what Java did to C/C+ when Java was at its peak. Back in those days many applications which needed performance did still use C/C+ but accompanied by high maintenance costs from the sheer amount of code written to achieve simple tasks and also the difficulty in building OS and machine independent applications. However, things changed in favor of Java when computers became powerful and hardware costs came down. People (like me) who looked down on using Java to build applications started accepting Java's advantages in building applications. Back then Python was primarily used as a scripting tool and was used in building test automation (and it still is being used for that purpose too.). Now comes the glorious age of data science and various tech startups in e-commerce and social space. What could be done by writing a thousand lines Java code is achieved in a couple of lines of code in Python. This means that there is less maintenance of code. On top of this, Python does not have arcane syntax and the code is pretty much human readable. For machine learning applications there are very good libraries such as Pandas, sci-kit-learn, etc. The primary advantage with Python is that focus can be put on solving business problems rather than spending the most time on fixing bugs related to faulty code. Coming to web development frameworks such as Django, Tornado provides a wide range of needs from quick development to high-performance websites.

Python is the older of the two languages. In programming language evolution, it is common to maintain backward compatibility indefinitely. This is not the case with Python. Python versions are largely compatible but have enough functionality- and syntax-breaking differences that they can be treated as different languages. Python is adapted because of Simplicity, Open source, object-oriented and General purpose language. It is an interpreted language available on many platforms, making it a portable option for general development.
Java on the other hand
Its attraction and value is its portability and relative efficiency. Java is a compiled language, unlike C/C+. Its execution model is more machine-centered than purely interpreted languages. Java is more than just a language and libraries. It is also a virtual machine and, therefore, an ecosystem. The Java Virtual Machine (JVM) is an idealized and portable platform for running Java code. Rather than worrying about hardware specifics and having to port code to new platforms, the promise of Java has been written once, run anywhere. That is so that as long as a JVM is present, anything compiled into its bytecode can run and interact easily with anything else written for the JVM.