Difference between revisions of "Python"
m (→Variables) |
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==getopt== | ==getopt== | ||
Module to parse the commandline arguments (sys.argv). | Module to parse the commandline arguments (sys.argv). | ||
+ | |||
+ | ==time== | ||
+ | Time functions | ||
+ | ;time.sleep(3) | ||
+ | :Sleep for 3 seconds | ||
+ | <syntaxhighlight lang=python> | ||
+ | from time import sleep | ||
+ | sleep(3) | ||
+ | </syntaxhighlight> | ||
==subprocess== | ==subprocess== |
Revision as of 17:10, 9 December 2018
Generic things:
The Python style guide is available in [PEP 8]
Modules
Modules need to be imported into your program by the import
command.
To add the location of your own modules to the python search path put it in the PYTHONPATH variable.\
- import <module>
- Import everything from the module, address components as <module>.<component>.
- import <module> as <short>
- Calls can have the short name. E.g. numpy is often imported as np
- from <module> import *
- Module components can be called without the module name. Beware of duplicates.
- from <module> import <component>
- Import a specific component from a modules, callable by just the component name.
We try to use modules that are available by default (on linux systems). If not it will be mentioned in the article.
sys
Provides a number of system variables
- sys.argv
- List of everything on the commandline. sys.argv[0] is the program itself.
getopt
Module to parse the commandline arguments (sys.argv).
time
Time functions
- time.sleep(3)
- Sleep for 3 seconds
from time import sleep
sleep(3)
subprocess
Module to execute shell commands
In python2
import subprocess
exitcode = subprocess.call("<any command>")
commandoutput = subprocess.check_output("<any command>")
Use ("command",shell=True) to have the call work like it would on the commandline
In python3
import subprocess
CompletedProcess = subprocess.run("<any command>")
The CompletedProcess returned has (args, returncode, stdout, stderr)
numpy
Non-default
Numpy is a module for scientific calculation. Introducing multi-dimensional arrays and more.
scipy
Non-default
Scipy is a module having more scientific functions.
pint
Non-default
Pint is a module to handle units.
Variables
Variables are always pointers to objects.
a = 2
b = 2
Both a and b point to the same object (the immutable integer '2')
Variables are local by default. If a routine has any assignment to a variable it is local. If you have defined a variable outside a routine and need assignments to it in the routine you have to declare it global explicitly.
a = 'a string'
def main():
global a
print(a)
a = "This would fail with 'local variable 'a' referenced before assignment' if 'a' was not declared as global"
main()
[Geeks for Geeks] has as good page about this.