other:python:misc_by_jyp
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other:python:misc_by_jyp [2022/12/12 12:11] jypeter Added the "Storing objects and data in a file" section |
other:python:misc_by_jyp [2023/04/26 15:50] jypeter Started a Data represenation section |
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| Line 5: | Line 5: | ||
| </WRAP> | </WRAP> | ||
| - | ==== Reading/setting environments variables ==== | ||
| + | ===== Reading/setting environments variables ===== | ||
| <code>>>> os.environ['TMPDIR'] | <code>>>> os.environ['TMPDIR'] | ||
| Line 17: | Line 17: | ||
| </code> | </code> | ||
| - | ==== Generating (aka raising) an error ==== | + | |
| + | ===== Generating (aka raising) an error ===== | ||
| This will stop the script, unless it is called in a function, and the code calling the function explicitely catches and deals with errors | This will stop the script, unless it is called in a function, and the code calling the function explicitely catches and deals with errors | ||
| Line 25: | Line 26: | ||
| - | ==== Stopping a script ==== | + | ===== Stopping a script ===== |
| A user can use ''CTRL-C'' or ''kill'' to stop a script, or ''CTRL-Z'' to suspend it temporarily (use ''fg'' to resume a suspended script). The code below can be used by the script itself to interrupt its execution, instead of raising an error | A user can use ''CTRL-C'' or ''kill'' to stop a script, or ''CTRL-Z'' to suspend it temporarily (use ''fg'' to resume a suspended script). The code below can be used by the script itself to interrupt its execution, instead of raising an error | ||
| Line 31: | Line 32: | ||
| <code>sys.exit('Some optional message about why we are stopping')</code> | <code>sys.exit('Some optional message about why we are stopping')</code> | ||
| + | ===== Data representation ===== | ||
| - | ==== Checking if a file/directory is writable by the current user ==== | + | A few notes for a future section or page about about data representation (bits and bytes) on disk and in memory, vs data format |
| + | |||
| + | * Binary data representation | ||
| + | * [[https://en.wikipedia.org/wiki/Bit_numbering|Bit numbering]] | ||
| + | * [[https://en.wikipedia.org/wiki/Endianness|Endianness]] | ||
| + | * [[https://en.wikipedia.org/wiki/Integer_(computer_science)|Integers]] | ||
| + | * Using [[https://en.wikipedia.org/wiki/Two%27s_complement|two's complement]] for negative integers | ||
| + | * Range: | ||
| + | * 4-byte integers: −2,147,483,648 to 2,147,483,647 | ||
| + | * 8-byte integers: −9,223,372,036,854,775,808 to 9,223,372,036,854,775,807 | ||
| + | * [[https://en.wikipedia.org/wiki/IEEE_754|Floating point numbers]] (//IEEE 754// standard) | ||
| + | * Range: | ||
| + | * 4-byte float: ~8 significant digits ^10E±38 | ||
| + | * See also [[https://en.wikipedia.org/wiki/Single-precision_floating-point_format|Single-precision floating-point format|Single-precision floating-point format]] | ||
| + | * 8-byte float: ~15 significant digits ^10E±308 | ||
| + | |||
| + | * Array addressing | ||
| + | |||
| + | * disk and ram usage: how to check the usage (available ram and disk), best practice on multi-user systems (how much allowed?) | ||
| + | * ''du'', ''df'', ''cat /proc/meminfo'', ''top'' | ||
| + | |||
| + | * understanding and reverse-engineering //binary// format | ||
| + | * ''od'', ''strings'' | ||
| + | |||
| + | * binary vs text format: ascii, utf, raw | ||
| + | * text related functions in python: ''str'', ''int'', ''float'', ''ord'', ... | ||
| + | * lists conversion with ''map'' and ''join'' | ||
| + | |||
| + | * Misc : ''md5sum'' | ||
| + | ===== Checking if a file/directory is writable by the current user ===== | ||
| <code>>>> os.access('/', os.W_OK) | <code>>>> os.access('/', os.W_OK) | ||
| Line 39: | Line 70: | ||
| True</code> | True</code> | ||
| - | ==== Playing with strings ==== | ||
| - | === Filenames, etc... === | + | ===== Playing with strings ===== |
| - | Check [[other:python:misc_by_jyp#working_with_paths_and_filenames|Working with paths and filenames]] and [[other:python:misc_by_jyp#generating_file_names|Generating file names]] | ||
| - | === Splitting strings === | + | ==== Splitting (complex) strings ==== |
| It's easy to split a string with multiple blank delimiters, or a specific delimiter, but it can be harder to deal with sub-strings | It's easy to split a string with multiple blank delimiters, or a specific delimiter, but it can be harder to deal with sub-strings | ||
| Line 64: | Line 93: | ||
| >>> shlex.split(complex_string) | >>> shlex.split(complex_string) | ||
| ['-o', '1', '--long', 'A string with accented chars: \xc3\xa9 \xc3\xa8 \xc3\xa0 \xc3\xa7']</code> | ['-o', '1', '--long', 'A string with accented chars: \xc3\xa9 \xc3\xa8 \xc3\xa0 \xc3\xa7']</code> | ||
| + | |||
| + | |||
| ==== Working with paths and filenames ==== | ==== Working with paths and filenames ==== | ||
| Line 149: | Line 180: | ||
| >>> f_tmp.close() | >>> f_tmp.close() | ||
| >>> os.remove(f_tmp.name)</code> | >>> os.remove(f_tmp.name)</code> | ||
| - | ==== Using command-line arguments ==== | ||
| - | === The extremely easy but non-flexible way: sys.argv === | + | |
| + | ===== Using command-line arguments ===== | ||
| + | |||
| + | ==== The extremely easy but non-flexible way: sys.argv ==== | ||
| The name of a script, the number of arguments (including the name of the script), and the arguments (as strings) can be accessed through the ''sys.argv'' strings' list | The name of a script, the number of arguments (including the name of the script), and the arguments (as strings) can be accessed through the ''sys.argv'' strings' list | ||
| Line 173: | Line 206: | ||
| 2 tas_tes.nc</code> | 2 tas_tes.nc</code> | ||
| - | === The C-style way: getopt === | + | |
| + | ==== The C-style way: getopt ==== | ||
| Use [[https://docs.python.org/3/library/getopt.html|getopt]] (//C-style parser for command line options//) | Use [[https://docs.python.org/3/library/getopt.html|getopt]] (//C-style parser for command line options//) | ||
| - | === The deprecated Python way: optparse === | + | |
| + | ==== The deprecated Python way: optparse ==== | ||
| [[https://docs.python.org/3/library/optparse.html|optparse]] (//parser for command line options//) is **deprecated since Python version 3.2**! You should now use argparse (check [[https://docs.python.org/3/library/argparse.html#upgrading-optparse-code|Upgrading optparse code]] for converting from ''optparse'' to ''argparse'') | [[https://docs.python.org/3/library/optparse.html|optparse]] (//parser for command line options//) is **deprecated since Python version 3.2**! You should now use argparse (check [[https://docs.python.org/3/library/argparse.html#upgrading-optparse-code|Upgrading optparse code]] for converting from ''optparse'' to ''argparse'') | ||
| - | === The current Python way: argparse === | + | |
| + | ==== The current Python way: argparse ==== | ||
| [[https://docs.python.org/3/library/argparse.html|argparse]] (//parser for command-line options, arguments and sub-commands//) is available since Python version 3.2 | [[https://docs.python.org/3/library/argparse.html|argparse]] (//parser for command-line options, arguments and sub-commands//) is available since Python version 3.2 | ||
| - | ==== Using ordered dictionaries ==== | + | |
| + | ===== Using ordered dictionaries ===== | ||
| **Dictionary order is guaranteed to be insertion order**! Note that the [[https://docs.python.org/3/library/stdtypes.html#dict|usual Python dictionary]] also guarantees the order since version **3.6** | **Dictionary order is guaranteed to be insertion order**! Note that the [[https://docs.python.org/3/library/stdtypes.html#dict|usual Python dictionary]] also guarantees the order since version **3.6** | ||
| Line 191: | Line 228: | ||
| Check the [[https://docs.python.org/3/library/collections.html#collections.OrderedDict|OrderedDict class]] (''from collections import OrderedDict'') and the [[https://realpython.com/python-ordereddict/|OrderedDict vs dict in Python: The Right Tool for the Job]] tutorial | Check the [[https://docs.python.org/3/library/collections.html#collections.OrderedDict|OrderedDict class]] (''from collections import OrderedDict'') and the [[https://realpython.com/python-ordereddict/|OrderedDict vs dict in Python: The Right Tool for the Job]] tutorial | ||
| - | ==== Using sets ==== | + | |
| + | ===== Using sets ===== | ||
| [[https://docs.python.org/3/tutorial/datastructures.html#sets|Python sets]] are **groups of unique elements**. They can be used to easily find all the unique elements of //something// and you can easily determine the **intersection**, **union** (and other similar operations) of sets. | [[https://docs.python.org/3/tutorial/datastructures.html#sets|Python sets]] are **groups of unique elements**. They can be used to easily find all the unique elements of //something// and you can easily determine the **intersection**, **union** (and other similar operations) of sets. | ||
| - | ==== Printing a readable version of long lists or dictionaries ==== | + | |
| + | ===== Printing a readable version of long lists or dictionaries ===== | ||
| The [[https://docs.python.org/3/library/pprint.html|pprint]] module can be used for //pretty printing// objects (lists, dictionaries, ...). It will wrap long lines in a meaningful way | The [[https://docs.python.org/3/library/pprint.html|pprint]] module can be used for //pretty printing// objects (lists, dictionaries, ...). It will wrap long lines in a meaningful way | ||
| Line 229: | Line 268: | ||
| </code> | </code> | ||
| - | ==== Storing objects and data in a file (shelve and friends) ==== | ||
| - | The built-in [[other:python:jyp_steps?s[]=shelve#the_shelve_package|shelve]] module can be **easily** used for storing temporary/intermediate data | + | ===== Storing objects and data in a file (shelve and friends) ===== |
| + | |||
| + | The built-in [[other:python:jyp_steps#the_shelve_package|shelve]] module can be **easily** used for storing temporary/intermediate data | ||
| More options: | More options: | ||
| Line 237: | Line 277: | ||
| * Working with [[other:python:jyp_steps#netcdf_filesusing_cdms2_xarray_and_netcdf4|NetCDF]] files | * Working with [[other:python:jyp_steps#netcdf_filesusing_cdms2_xarray_and_netcdf4|NetCDF]] files | ||
| - | ==== Sorting ==== | + | |
| + | ===== Using a configuration file ===== | ||
| + | |||
| + | The built-in [[https://docs.python.org/3/library/configparser.html|configparser]] module can be easily used for reading (**and** writing!) text configuration files. | ||
| + | |||
| + | Note: a configuration file is also a way to easily store and exchange text data ! | ||
| + | |||
| + | |||
| + | ===== Working with global variables ===== | ||
| + | |||
| + | There is a good chance you don't actually want/need a //global// variable. Be sure to use the ''global'' statement correctly if you want to avoid side-effects... | ||
| + | |||
| + | * [[https://docs.python.org/3/faq/programming.html?highlight=global#why-am-i-getting-an-unboundlocalerror-when-the-variable-has-a-value|Using (and changing) a global variable inside a script or module]] | ||
| + | * Simple module example\\ <code>_myvar = 10 | ||
| + | |||
| + | def set_myvar(new_val): | ||
| + | # Note: need to explicitly define a global variable (of a module) | ||
| + | # as 'global' BEFORE changing its value in a function! | ||
| + | # Otherwise, the value will not be REdefined outside the function | ||
| + | global _myvar | ||
| + | _myvar = new_val | ||
| + | |||
| + | def get_myvar(): | ||
| + | return _myvar | ||
| + | |||
| + | def myfunc(nb_repeat = 10): | ||
| + | print(nb_repeat * _myvar)</code> | ||
| + | * [[https://docs.python.org/3/faq/programming.html?highlight=global#how-do-i-share-global-variables-across-modules|Sharing global variables across modules]] | ||
| + | ===== Sorting ===== | ||
| * When dealing with **numerical values**, you should use the [[https://numpy.org/doc/stable/reference/routines.sort.html|numpy sorting, searching, and counting routines]]! | * When dealing with **numerical values**, you should use the [[https://numpy.org/doc/stable/reference/routines.sort.html|numpy sorting, searching, and counting routines]]! | ||
| Line 254: | Line 322: | ||
| ['c', 'd', 'b', 'a']</code> | ['c', 'd', 'b', 'a']</code> | ||
| - | ==== numpy related stuff ==== | + | ===== numpy related stuff ===== |
| - | === Using a numpy array to store arbitrary objects === | + | ==== Using a numpy array to store arbitrary objects ==== |
| The numpy arrays are usually used to store [[https://numpy.org/doc/stable/reference/arrays.scalars.html|scalars]] of the same type (see also the [[https://numpy.org/doc/stable/reference/arrays.dtypes.html|Data type objects (dtype)]]), very often numerical values. | The numpy arrays are usually used to store [[https://numpy.org/doc/stable/reference/arrays.scalars.html|scalars]] of the same type (see also the [[https://numpy.org/doc/stable/reference/arrays.dtypes.html|Data type objects (dtype)]]), very often numerical values. | ||
| Line 275: | Line 343: | ||
| [<cartopy.mpl.contour.GeoContourSet object at 0x2ab679e8bf10>, | [<cartopy.mpl.contour.GeoContourSet object at 0x2ab679e8bf10>, | ||
| None, None]], dtype=object)</code> | None, None]], dtype=object)</code> | ||
| + | |||
| | | ||
| - | === Dealing with a variable number of indices === | + | ==== Dealing with a variable number of indices ==== |
| [[https://numpy.org/doc/stable/user/basics.indexing.html#dealing-with-variable-indices|Official reference]] | [[https://numpy.org/doc/stable/user/basics.indexing.html#dealing-with-variable-indices|Official reference]] | ||
| Line 340: | Line 409: | ||
| [ 0., 0., 0., 0., 0., 0., 0., 0., 0., 1.]])</code> | [ 0., 0., 0., 0., 0., 0., 0., 0., 0., 1.]])</code> | ||
| - | === Finding and counting unique values === | + | |
| + | ==== Finding and counting unique values ==== | ||
| Use ''np.unique'', do **not** try to use histogram related functions! | Use ''np.unique'', do **not** try to use histogram related functions! | ||
| Line 360: | Line 430: | ||
| array([1. , 1. , 1. , 1.5, 1.5, 1.5, 2. , 2. , 2. , 2. ])</code> | array([1. , 1. , 1. , 1.5, 1.5, 1.5, 2. , 2. , 2. , 2. ])</code> | ||
| - | === Applying a ufunc over all the elements of an array === | + | |
| + | ==== Applying a ufunc over all the elements of an array ==== | ||
| There are all sorts of //ufuncs// (Universal Functions), and we will just use below ''add'' from the [[https://numpy.org/doc/stable/reference/ufuncs.html#math-operations|math operations]], applied on the arrays defined in [[#finding_and_counting_unique_values|Finding and counting unique values]] | There are all sorts of //ufuncs// (Universal Functions), and we will just use below ''add'' from the [[https://numpy.org/doc/stable/reference/ufuncs.html#math-operations|math operations]], applied on the arrays defined in [[#finding_and_counting_unique_values|Finding and counting unique values]] | ||
| Line 393: | Line 464: | ||
| (3.0, 4.5, 8.0)</code> | (3.0, 4.5, 8.0)</code> | ||
| - | === Applying a ufunc over specified sections of an array === | + | |
| + | ==== Applying a ufunc over specified sections of an array ==== | ||
| The [[https://numpy.org/doc/stable/reference/generated/numpy.ufunc.reduceat.html#numpy.ufunc.reduceat|reduceat]] function can be used to avoid explicit python loops, and improve the speed (but not the readability...) of a script. The example below //improves// what has been shown above | The [[https://numpy.org/doc/stable/reference/generated/numpy.ufunc.reduceat.html#numpy.ufunc.reduceat|reduceat]] function can be used to avoid explicit python loops, and improve the speed (but not the readability...) of a script. The example below //improves// what has been shown above | ||
| Line 411: | Line 483: | ||
| >>> np.add.reduceat(np.sort(vals), slices_indices) | >>> np.add.reduceat(np.sort(vals), slices_indices) | ||
| array([3. , 4.5, 8. ])</code> | array([3. , 4.5, 8. ])</code> | ||
| + | |||
| + | |||
| + | ===== matplotlib related stuff ===== | ||
| + | |||
| + | ==== Working with time axes (and ticks) ==== | ||
| + | |||
| + | If you have problems setting the limits of a time axis, choosing the ticks' locations, or specifying the style of the labels, you should check the: | ||
| + | * [[https://matplotlib.org/stable/gallery/index.html#ticks|Ticks examples' gallery]] | ||
| + | * [[https://matplotlib.org/stable/gallery/text_labels_and_annotations/date.html|Date tick labels example]] | ||
| + | |||
| /* | /* | ||
| - | ==== Tip template ==== | + | ===== Tip template ===== |
| <code>Some code</code> | <code>Some code</code> | ||
other/python/misc_by_jyp.txt · Last modified: 2024/04/19 12:02 by jypeter
