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Python cheat

Some Python hints for myself as a Python novice. Especially differences to C/C#/Java ...

See: Coding Conventions for a big mess ...

Table of Contents

Data Types

Types and Casting

x = "3"
y = 3
z = 3.0

x = str(3)
y = int(3)
z = float(3)

print(type(x))

Global

def func():
    global a
    a = "fantastic"

func()
print("Global: " + a)

Strings

Various methods: https://www.w3schools.com/python/python_strings_methods.asp

Creation

r"C:\abc"
f"text with {variable}"
f"text with {variable:.2f}"		# two decimals of a float
fr"C:\text\{variable}"
  • r: text ignoring escape chars (especially for Windows paths)
  • f: format variable content into the string

Links:

Checks

Contains and StartsWith:

if "string" in myString:
    ...

if myString.startswith(prefix):
    ...

Split

for line in textBlob:
    print(line)

text[1:3] # including 1, excluding 3
text[:4]  # from start, excluding 4
text[4:]  # including 4 to the end

text = "a, b"
print(a.split(",")) 	# returns ['a', ' b']

Changes

text.upper()
text.lower()
text.strip()	# remove leading and trailing spaces
text.replace("a", "b")

Lists (Arrays)

some_names = [
    "Alice",
    "Bob",
]

list_in_list = [
    ["Alice", "Merton"],
    ["Bob", "Hope"],
]

alice = some_names[0]
n = len(some_names)
for name in some_names:
    print(name)
some_names.pop(0) # removes first item
some_names.remove("Alice")

Other list/array actions: https://www.w3schools.com/python/python_arrays.asp

Dictionaries

Key:Value store.

person = {
    "first": "Alice",
    "name": "Merton",
    "year": 1993
}
person = dict(first = "Alice", name = "Merton", year = 1993)

print(person)
print(person["name"])
print(len(person))

https://www.w3schools.com/python/python_dictionaries.asp

Control Flow

if

if a==b and not c==d or e!=f:
    print(a)
elif not a==b:
    print(b)
else:
    print(c)

switch (match/case)

match variable:
    case "a" | "b":
        # do a
    case _:
        # default

return

return a, b, c

Exceptions

try:
    # action
except PermissionError:
    # reaction
except OSError as error:
    if error.errno == 41:
        # more ...
except Exception as error:
    # more ...

Terminate Program

exit(1)

Unfortunately, there is no exit code standard ... https://stackoverflow.com/questions/1101957/are-there-any-standard-exit-status-codes-in-linux

Maybe some common ones:

  • 0: ok
  • 1: general error
  • 2: command line usage error
  • x: other failure ...

Files and Folders

import os

Infos

os.path.exists(dirOrFile)
os.path.isdir(dir)
os.path.isfile(file)
os.path.getsize(file)

joinedPath = os.path.join(dir, file)
extension = os.path.splitext(file)[1]

Current Dir

cwd = os.getcwd()
os.chdir(dir)

Iterate

for dirsAndFiles in os.listdir(dir):
    ...

Manipulate

os.makedirs(dir)
os.remove(file)
os.rmdir(dir)

Input / Output

with open("infile.txt", 'r', encoding='utf8') as infile:
    with open("outfile.txt", 'w', encoding='utf8') as outfile:
        while line := infile.readline():
            outfile.write("pre-" + line + "\n")

File modes: https://www.w3schools.com/python/python_file_handling.asp

System

Sleep

import time
time.sleep(100/1000)    # sleep 100 ms

Command Line Parser

Parsing command line arguments:

import argparse
if __name__ == '__main__':
    parser = argparse.ArgumentParser(formatter_class=argparse.ArgumentDefaultsHelpFormatter)
    parser.add_argument('--input', '-i', help='input file', required="True")
    parser.add_argument('--verbose', '-v', help='show verbose', action='store_true', dest="show_verbose")

    parser = parser.parse_args(sys.argv[1:])
    if parser.show_verbose:
        print(parser.input)

Environment Variables

import os
userprofile = os.environ['USERPROFILE']

Machine Name

import platform
name = platform.node()

Check for Running Debugger

Not perfect, will probably only work with VS Code:

isDebuggerRunning = os.environ.get('DEBUGPY_RUNNING') == "true"

Console

Console Output

print("\rtext", end='', flush=True)
  • \r "carriage return" will point cursor at the start of the console line
  • end='' avoids a newline (console won't jump to the next line)
  • flush=True prints out immediately on (Windows) console (as end='' causes the output to be buffered and not shown immediately)

Console Input

Wait for "enter key" pressed:

text = input("Enter a text:")

Wait for single "any key" press (Windows only, there seems to be no easy portable way)

import msvcrt
selected = msvcrt.getch().decode('ASCII')

Set Console Title

Set the title of the console window:

system("title " + title)

Set Console Colors

https://stackoverflow.com/questions/287871/how-do-i-print-colored-text-to-the-terminal

CRED = '\33[31m'
CGREEN = '\33[32m'
CBLUE = '\33[34m'
CEND = '\33[0m'
print(CRED + "red" + CEND)
print(CGREEN + "green" + CEND)
print(CBLUE + "blue" + CEND)

Web

import urllib.request
urllib.request.urlopen("url").read()

Open Office (pandas)

Use pandas for handling of data e.g. in LibreOffice .ods files.

https://www.w3schools.com/python/pandas/default.asp

Install pandas

pip install pandas

TODO: use venv

Iterate rows

import pandas as pd

df = pd.read_excel("test.ods", "sheet", engine="odf")
print(df)
for index, row in df.iterrows():
    contentFromCell = str(row["Column Header"])

External Processes

Process Execution with Wait

subprocess.run will wait until the child process has finished.

import subprocess

Simple Execution

command = "doit parameter"
result = subprocess.run(command, stdout=sys.stdout)
returned = result.returncode

Output to String

command = "choco outdated"
result = subprocess.run(command.split(), capture_output=True, text=True)
returned = result.returncode
for line in result.stdout.splitlines():
    ...

Output to File

command = f"choco list -lo -r -y"
with open("output.txt", 'w', encoding='utf8') as outfile:
    subprocess.run(command, stdout=outfile, stderr=subprocess.STDOUT)

Process Execution without Wait

The DETACHED_PROCESS flag is Windows specific! There seems to be no easy platform independent way to do this 👎

subprocess.Popen(start, creationflags=subprocess.DETACHED_PROCESS)

⚠️ Warning: If running in a debugger, closing the debug session will stop all processes created here!

Process Manipulation (psutils)

Platform independent process handling.

Install psutils

pip install psutil

TODO: use venv

Search for Process

for proc in psutil.process_iter(attrs=['pid', 'name']):
    if proc.info['name'] == "explorer.exe":
        ...

Terminate Process

proc.terminate()
proc.kill()

Terminate is gracefully shutdown, kill is hard kill.