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Python Syntax

Introduction

A Python program is read by a parser. Python was designed to be a highly readable language. The syntax of the Python programming language is the set of rules which defines how a Python program will be written.

Python Line Structure:

A Python program is divided into a number of logical lines and every logical line is terminated by the token NEWLINE. A logical line is created from one or more physical lines.
A line contains only spaces, tabs, formfeeds possibly a comment, is known as a blank line, and Python interpreter ignores it.
A physical line is a sequence of characters terminated by an end-of-line sequence (in windows it is called CR LF or return followed by a linefeed and in Unix, it is called LF or linefeed). See the following example.

Python Line structure

 

Comments in Python:

A comment begins with a hash character(#) which is not a part of the string literal and ends at the end of the physical line. All characters after the # character up to the end of the line are part of the comment and the Python interpreter ignores them. See the following example. It should be noted that Python has no multi-lines or block comments facility.

Python Comment

 

Joining two lines:

When you want to write a long code in a single line you can break the logical line in two or more physical lines using backslash character(\). Therefore when a physical line ends with a backslash characters(\) and not a part of a string literal or comment then it can join another physical line. See the following example.

Python lines breaking rule

 

Multiple Statements on a Single Line:

You can write two separate statements into a single line using a semicolon (;) character between two line.

Python multiple statement into a single line

 

Indentation:

Python uses whitespace (spaces and tabs) to define program blocks whereas other languages like C, C++ use braces ({}) to indicate blocks of codes for class, functions or flow control. The number of whitespaces (spaces and tabs) in the indentation is not fixed, but all statements within the block must be the indented same amount. In the following program, the block statements have no indentation.

Python statements without indentation

 

This is a program with single space indentation.

Python single space indentation

 

This is a program with single tab indentation.

Python single tab indentation example

 

Here is an another program with an indentation of a single space + a single tab.

Python single space and -tab indentation

 

Python Coding Style:

  • Use 4 spaces per indentation and no tabs.
  • Do not mix tabs and spaces. Tabs create confusion and it is recommended to use only spaces.
  • Maximum line length : 79 characters which help users with a small display.
  • Use blank lines to separate top-level function and class definitions and single blank line to separate methods definitions inside a class and larger blocks of code inside functions.
  • When possible, put inline comments (should be complete sentences).
  • Use spaces around expressions and statements.

Python Reserve words:

The following identifiers are used as reserved words of the language, and cannot be used as ordinary identifiers.

False class finally is return
None continue for lambda try
True def from nonlocal while
and del global not with
as el if or yield
assert else import pass  
break except in raise  

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Python: Tips of the Day

Python: How to install pip on Windows?

Python 2.7.9+ and 3.4+

Good news! Python 3.4 (released March 2014) and Python 2.7.9 (released December 2014) ship with Pip. This is the best feature of any Python release. It makes the community's wealth of libraries accessible to everyone. Newbies are no longer excluded from using community libraries by the prohibitive difficulty of setup. In shipping with a package manager, Python joins Ruby, Node.js, Haskell, Perl, Go-almost every other contemporary language with a majority open-source community. Thank you, Python.

If you do find that pip is not available when using Python 3.4+ or Python 2.7.9+, simply execute e.g.:

py -3 -m ensurepip

Of course, that doesn't mean Python packaging is problem solved. The experience remains frustrating. I discuss this in the Stack Overflow question Does Python have a package/module management system?.

And, alas for everyone using Python 2.7.8 or earlier (a sizable portion of the community). There's no plan to ship Pip to you. Manual instructions follow.

Python 2 = 2.7.8 and Python 3 = 3.3

Flying in the face of its 'batteries included' motto, Python ships without a package manager. To make matters worse, Pip was-until recently-ironically difficult to install.

Official instructions

Per https://pip.pypa.io/en/stable/installing/#do-i-need-to-install-pip:

Download get-pip.py, being careful to save it as a .py file rather than .txt. Then, run it from the command prompt:

python get-pip.py

You possibly need an administrator command prompt to do this. Follow Start a Command Prompt as an Administrator (Microsoft TechNet).

This installs the pip package, which (in Windows) contains ...\Scripts\pip.exe that path must be in PATH environment variable to use pip from the command line (see the second part of 'Alternative Instructions' for adding it to your PATH,

Alternative instructions

The official documentation tells users to install Pip and each of its dependencies from source. That's tedious for the experienced and prohibitively difficult for newbies.

For our sake, Christoph Gohlke prepares Windows installers (.msi) for popular Python packages. He builds installers for all Python versions, both 32 and 64 bit. You need to:

  1. Install setuptools
  2. Install pip

For me, this installed Pip at C:\Python27\Scripts\pip.exe. Find pip.exe on your computer, then add its folder (for example, C:\Python27\Scripts) to your path (Start / Edit environment variables). Now you should be able to run pip from the command line. Try installing a package:

pip install httpie

There you go (hopefully)! Solutions for common problems are given below:

Proxy problems

If you work in an office, you might be behind an HTTP proxy. If so, set the environment variables http_proxy and https_proxy. Most Python applications (and other free software) respect these. Example syntax:

http://proxy_url:port
http://username:[email protected]_url:port

If you're really unlucky, your proxy might be a Microsoft NTLM proxy. Free software can't cope. The only solution is to install a free software friendly proxy that forwards to the nasty proxy. http://cntlm.sourceforge.net/

Unable to find vcvarsall.bat

Python modules can be partly written in C or C++. Pip tries to compile from source. If you don't have a C/C++ compiler installed and configured, you'll see this cryptic error message.

Error: Unable to find vcvarsall.bat

You can fix that by installing a C++ compiler such as MinGW or Visual C++. Microsoft actually ships one specifically for use with Python. Or try Microsoft Visual C++ Compiler for Python 2.7.

Often though it's easier to check Christoph's site for your package.

Ref: https://bit.ly/2B0ch3y