Python
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 finds a file <modulename>.py or __init__.py in directory <modulename>. <modulename>.py or __init__.py are executed on import. Usually not many code is in modules to execute immediately, functions and classes are mostly in there.
- 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 this article.
Only modules for which we use a very limited number of functions are listed here. More complex modules have there own article
sys
Provides a number of system variables
- sys.argv
- List of everything on the commandline. sys.argv[0] is the program itself.
- sys.version
- The python version you run
- sys.path
- The directories python looks into when doing an import. The script location always in sys.path. Directories in the environment variable $PYTHONPATH are added to sys.path
datetime
Date and time functions
from datetime import datetime
timestamp = datetime.now().strftime("%Y%m%d_%H%M%S")
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)
random
Generate random numbers.
- random.random()
- Return a floating point in the range from 0.0 to 1.0 (including both)
- random.randint(start,stop)
- Return an integer in the range from start to stop (including both)
- Pick a random element from a list:
list[random.randint(0,len(list)-1)]
list[int(random.random()*len(list))]
threading
Enable parallel processing
- t1=threading.Thread(target=<a function>)
- Return a thread object to run <a function> in the background
- t1.start()
- Start the thread for the function targeted by t1
- t1.join(<timeout>)
- Wait until t1 is ready or until <timeout> has expired. Returns None always.
- t1.is_alive()
- Return True if t1 is still running (usefull e.g. after join with timeout).
getopt
Module to parse the commandline arguments (sys.argv).
Variables
Everything is an object in python. Objects can be variables and functions.
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.