Getting started with Python LanguageList comprehensionsFilterListFunctionsDecoratorsMath ModuleLoopsRandom moduleComparisonsImporting modulesSorting, Minimum and MaximumOperator moduleVariable Scope and BindingBasic Input and OutputFiles & Folders I/OJSON ModuleString MethodsMetaclassesIndexing and SlicingGeneratorsSimple Mathematical OperatorsReduceMap FunctionExponentiationSearchingDictionaryClassesCountingManipulating XMLDate and TimeSetCollections moduleParallel computationMultithreadingWriting extensionsUnit TestingRegular Expressions (Regex)Bitwise OperatorsIncompatibilities moving from Python 2 to Python 3Virtual environmentsCopying dataTupleContext Managers (“with” Statement)Hidden FeaturesEnumString FormattingConditionalsComplex mathUnicode and bytesThe __name__ special variableChecking Path Existence and PermissionsPython NetworkingAsyncio ModuleThe Print Functionos.pathCreating Python packagesParsing Command Line argumentsHTML ParsingSubprocess Librarysetup.pyList slicing (selecting parts of lists)SocketsItertools ModuleRecursionBoolean OperatorsThe dis moduleType Hintspip: PyPI Package ManagerThe locale ModuleExceptionsWeb scraping with PythonDeque ModuleDistributionProperty ObjectsOverloadingDebuggingReading and Writing CSVDynamic code execution with `exec` and `eval`PyInstaller - Distributing Python CodeIterables and IteratorsData Visualization with PythonThe Interpreter (Command Line Console)*args and **kwargsFunctools ModuleGarbage CollectionIndentationSecurity and CryptographyPickle data serialisationurllibBinary DataPython and ExcelIdiomsMethod OverridingDifference between Module and PackageData SerializationPython concurrencyIntroduction to RabbitMQ using AMQPStormPostgreSQLDescriptorCommon PitfallsMultiprocessingtempfile NamedTemporaryFileWorking with ZIP archivesStackProfilingUser-Defined MethodsWorking around the Global Interpreter Lock (GIL)DeploymentLoggingProcesses and ThreadsThe os ModuleComments and DocumentationDatabase AccessPython HTTP ServerAlternatives to switch statement from other languagesList destructuring (aka packing and unpacking)Accessing Python source code and bytecodeMixinsAttribute AccessArcPyPython Anti-PatternsPlugin and Extension ClassesWebsocketsImmutable datatypes(int, float, str, tuple and frozensets)String representations of class instances: __str__ and __repr__ methodsArraysOperator PrecedencePolymorphismNon-official Python implementationsList ComprehensionsWeb Server Gateway Interface (WSGI)2to3 toolAbstract syntax treeAbstract Base Classes (abc)UnicodeSecure Shell Connection in PythonPython Serial Communication (pyserial)Neo4j and Cypher using Py2NeoBasic Curses with PythonPerformance optimizationTemplates in pythonPillowThe pass statementLinked List Nodepy.testDate FormattingHeapqtkinterCLI subcommands with precise help outputDefining functions with list argumentsSqlite3 ModulePython PersistenceTurtle GraphicsConnecting Python to SQL ServerDesign PatternsMultidimensional arraysAudioPygletQueue ModuleijsonWebbrowser ModuleThe base64 ModuleFlaskgroupby()Sockets And Message Encryption/Decryption Between Client and ServerpygameInput, Subset and Output External Data Files using Pandashashlibgetting start with GZipDjangoctypesCreating a Windows service using PythonPython Server Sent EventsMutable vs Immutable (and Hashable) in PythonPython speed of programconfigparserLinked listsCommonwealth ExceptionsOptical Character RecognitionPython Data TypesPartial functionspyautogui modulegraph-toolUnzipping FilesFunctional Programming in PythonPython Virtual Environment - virtualenvsysvirtual environment with virtualenvwrapperCreate virtual environment with virtualenvwrapper in windowsPython Requests PostPlotting with MatplotlibPython Lex-YaccChemPy - python packagepyaudioshelveUsage of "pip" module: PyPI Package ManagerIoT Programming with Python and Raspberry PICode blocks, execution frames, and namespaceskivy - Cross-platform Python Framework for NUI DevelopmentCall Python from C#Similarities in syntax, Differences in meaning: Python vs. JavaScriptWriting to CSV from String or ListRaise Custom Errors / ExceptionsUsing loops within functionsPandas Transform: Preform operations on groups and concatenate the results

The Interpreter (Command Line Console)

Other topics

Getting general help

If the help function is called in the console without any arguments, Python presents an interactive help console, where you can find out about Python modules, symbols, keywords and more.

>>> help()

Welcome to Python 3.4's help utility!

If this is your first time using Python, you should definitely check out
the tutorial on the Internet at http://docs.python.org/3.4/tutorial/.

Enter the name of any module, keyword, or topic to get help on writing
Python programs and using Python modules.  To quit this help utility and
return to the interpreter, just type "quit".

To get a list of available modules, keywords, symbols, or topics, type
"modules", "keywords", "symbols", or "topics".  Each module also comes
with a one-line summary of what it does; to list the modules whose name
or summary contain a given string such as "spam", type "modules spam".

Referring to the last expression

To get the value of the last result from your last expression in the console, use an underscore _.

>>> 2 + 2
4
>>> _
4
>>> _ + 6
10

This magic underscore value is only updated when using a python expression that results in a value. Defining functions or for loops does not change the value. If the expression raises an exception there will be no changes to _.

>>> "Hello, {0}".format("World")
'Hello, World'
>>> _
'Hello, World'
>>> def wontchangethings():
...     pass
>>> _
'Hello, World'
>>> 27 / 0
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: division by zero
>>> _
'Hello, World'

Remember, this magic variable is only available in the interactive python interpreter. Running scripts will not do this.

Opening the Python console

The console for the primary version of Python can usually be opened by typing py into your windows console or python on other platforms.

$ py
Python 3.4.3 (v3.4.3:9b73f1c3e601, Feb 24 2015, 22:44:40) [MSC v.1600 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>>

If you have multiple versions, then by default their executables will be mapped to python2 or python3 respectively.

This of course depends on the Python executables being in your PATH.

The PYTHONSTARTUP variable

You can set an environment variable called PYTHONSTARTUP for Python's console. Whenever you enter the Python console, this file will be executed, allowing for you to add extra functionality to the console such as importing commonly-used modules automatically.

If the PYTHONSTARTUP variable was set to the location of a file containing this:

print("Welcome!")

Then opening the Python console would result in this extra output:

$ py
Python 3.4.3 (v3.4.3:9b73f1c3e601, Feb 24 2015, 22:44:40) [MSC v.1600 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
Welcome!
>>>    

Command line arguments

Python has a variety of command-line switches which can be passed to py. These can be found by performing py --help, which gives this output on Python 3.4:

Python Launcher

usage: py [ launcher-arguments ] [ python-arguments ] script [ script-arguments ]

Launcher arguments:

-2     : Launch the latest Python 2.x version
-3     : Launch the latest Python 3.x version
-X.Y   : Launch the specified Python version
-X.Y-32: Launch the specified 32bit Python version

The following help text is from Python:

usage: G:\Python34\python.exe [option] ... [-c cmd | -m mod | file | -] [arg] ...
Options and arguments (and corresponding environment variables):
-b     : issue warnings about str(bytes_instance), str(bytearray_instance)
         and comparing bytes/bytearray with str. (-bb: issue errors)
-B     : don't write .py[co] files on import; also PYTHONDONTWRITEBYTECODE=x
-c cmd : program passed in as string (terminates option list)
-d     : debug output from parser; also PYTHONDEBUG=x
-E     : ignore PYTHON* environment variables (such as PYTHONPATH)
-h     : print this help message and exit (also --help)
-i     : inspect interactively after running script; forces a prompt even
         if stdin does not appear to be a terminal; also PYTHONINSPECT=x
-I     : isolate Python from the user's environment (implies -E and -s)
-m mod : run library module as a script (terminates option list)
-O     : optimize generated bytecode slightly; also PYTHONOPTIMIZE=x
-OO    : remove doc-strings in addition to the -O optimizations
-q     : don't print version and copyright messages on interactive startup
-s     : don't add user site directory to sys.path; also PYTHONNOUSERSITE
-S     : don't imply 'import site' on initialization
-u     : unbuffered binary stdout and stderr, stdin always buffered;
         also PYTHONUNBUFFERED=x
         see man page for details on internal buffering relating to '-u'
-v     : verbose (trace import statements); also PYTHONVERBOSE=x
         can be supplied multiple times to increase verbosity
-V     : print the Python version number and exit (also --version)
-W arg : warning control; arg is action:message:category:module:lineno
         also PYTHONWARNINGS=arg
-x     : skip first line of source, allowing use of non-Unix forms of #!cmd
-X opt : set implementation-specific option
file   : program read from script file
-      : program read from stdin (default; interactive mode if a tty)
arg ...: arguments passed to program in sys.argv[1:]

Other environment variables:
PYTHONSTARTUP: file executed on interactive startup (no default)
PYTHONPATH   : ';'-separated list of directories prefixed to the
               default module search path.  The result is sys.path.
PYTHONHOME   : alternate <prefix> directory (or <prefix>;<exec_prefix>).
               The default module search path uses <prefix>\lib.
PYTHONCASEOK : ignore case in 'import' statements (Windows).
PYTHONIOENCODING: Encoding[:errors] used for stdin/stdout/stderr.
PYTHONFAULTHANDLER: dump the Python traceback on fatal errors.
PYTHONHASHSEED: if this variable is set to 'random', a random value is used
   to seed the hashes of str, bytes and datetime objects.  It can also be
   set to an integer in the range [0,4294967295] to get hash values with a
   predictable seed.

Getting help about an object

The Python console adds a new function, help, which can be used to get information about a function or object.

For a function, help prints its signature (arguments) and its docstring, if the function has one.

>>> help(print)
Help on built-in function print in module builtins:

print(...)
    print(value, ..., sep=' ', end='\n', file=sys.stdout, flush=False)

    Prints the values to a stream, or to sys.stdout by default.
    Optional keyword arguments:
    file:  a file-like object (stream); defaults to the current sys.stdout.
    sep:   string inserted between values, default a space.
    end:   string appended after the last value, default a newline.
    flush: whether to forcibly flush the stream.

For an object, help lists the object's docstring and the different member functions which the object has.

>>> x = 2
>>> help(x)
Help on int object:

class int(object)
 |  int(x=0) -> integer
 |  int(x, base=10) -> integer
 |
 |  Convert a number or string to an integer, or return 0 if no arguments
 |  are given.  If x is a number, return x.__int__().  For floating point
 |  numbers, this truncates towards zero.
 |
 |  If x is not a number or if base is given, then x must be a string,
 |  bytes, or bytearray instance representing an integer literal in the
 |  given base.  The literal can be preceded by '+' or '-' and be surrounded
 |  by whitespace.  The base defaults to 10.  Valid bases are 0 and 2-36.
 |  Base 0 means to interpret the base from the string as an integer literal.
 |  >>> int('0b100', base=0)
 |  4
 |
 |  Methods defined here:
 |
 |  __abs__(self, /)
 |      abs(self)
 |
 |  __add__(self, value, /)
 |      Return self+value...

Contributors

Topic Id: 2473

Example Ids: 8158,8159,8160,8161,8162,8163

This site is not affiliated with any of the contributors.