Python

Exception

try:
   raise Exception("not allowed")
except ValueError as error:
   print('value error: ' + repr(error))
except SyntaxError as error:
   print('syntax error: ' + repr(error))
else:
    raise Exception("unknown")
try:
   doit()
finally:
   closeIt()

Regular Expression

rexpr = compile(r"([\da-z]+)")
match = rexpr.match(line)
if match != None:
   number = (int) rexpr.group(1)

Datentypen

String

• "x" und 'x' sind gleichwertig
• Formatierung

"{:3.7f} {:s} {:07d}".format(3.14, "hello", 100)
"{0:d} / {0:x}".format(714)
"{}-{}: {}".format('abc.txt', 9, 7.99)

Container

• List: ['a', 1]; x[1] = 5; x.insert(0, 'firstItem'); x.remove('a'); ix=x.index('a'); del x[0];
• Tupel: t = ('a', 1); l = list(t)

Dictionary

x = { 'key' : 'val', 'xyz': 3 }
x['key'] = value
del x['key'];
contains = 'key' in x and 'key2' not in x
size = len(x)
for pair in x.iteritems():
  key = pair[0]
for key in x.iterkeys():
  print key

• x.itervalues()
• x.setdefault(key[, value]): setzt Wert nur, wenn noch nicht gesetzt
• x.keys(), x.values()
• x.copy(): flache Kopie
• x.update(dict): addiert dict zu x

Mengen

s = set(['y', 3]) ; f = frozenset(['y', 3]) 
for elem in s:
  print elem
size = len(s)
contains = 3 in s and 4 not in s
intersection = s & f
union = s | f
isPartOf = s <= f
diff = s - f

Datum/Zeit

import datetime, time
# ab hour optional:
date1 = datetime.datetime(2019, 4, 1, 22, 44, 12, 123456)
now = datetime.datetime.now()
daylySeconds = (now.time().hour*60+now.time().minute)*60+now.time().second
yesterday = datetime.datetime.fromtimestamp(time.time() - 24*3600)
firstOfMonth = date1.replace(day=1)
asStringMicroseconds = date1.strftime('%Y.%m.%dT%H:%M:%S.%f dayOfTheWeek: %w')

Typcheck

isStringOrSubclass = isinstance(aVariable, str)
isString = type(aVariable) is str
isList = type([1, 2]) is list
isDict = type({ 0:"a", 1:"b" }) is dict

Spezielle Methoden/Attribute

• Statische Methoden:

class X:
   # statische Variable
   _data = []
   @staticmethod
   def add(item):
      X._data.append(item)

X.add("new")

• Feststellen, ob Attribut existiert: hasattr(instance, nameOfAttribute)
• dynamischer Code:

exec 'import ' + module

• Vollständige Kopie (deep copy):

import copy
x = [1, 2]
y = copy.deepcopy(x)

• Superclass-Konstruktor:

class Parent:
   def __init__(self, name)
      self._name = name
class Child(Parent):
   def __init__(self, name):
      Parser.__init__(self, name)

Typische Situationen

Externes Programm aufrufen

with supbprocess.popen([ '/usr/bin/wc', '-l', file ], stdout=subprocess.PIPE) as proc:
   count = int(proc.stdout.read().decode())

Dateien

• Lesen:

with open(self._filename, "r") as fp:
  for line in fp:
     print(line)
  # fp.close() ist implizit

• Schreiben:

with open(self._filename, "w") as fp:
  fp.write("Hello\nWorld\n");

Sprachbesonderheiten

Verschachtelte Methoden

class Example:
  def scan(self, file):
     lineNo = 0
     for line in file:
       lineNo += 1
       if line.startswith('[':
           chapter = Chapter(line[1:-1])
       elif re.match(r'\w+='):
           var, value = line.split('=')
           chapter._vars[var] = value
       else:
           _error('invalid input')
      def _error(msg):
         print("line {}: {}\n{}".format(lineNo, msg, line)

Klasse als Sequenz

Damit eine Klasse mit "x in classInstance" angesprochen werden kann, muss es einen Iterator geben. Im Beispiel wird dies in einer Klasse zusammengefasst:__iter__() liefert als Iterator sich selbst und __next__() implementiert diesen Iterator:

class Example:
  def __init__():
    self._nextItems = []
  def __iter__():
    self._nextItems = [1, 2, 3]
    return self
  def __next__():
    if len(self._nextItem) == 0:
      raise StopIteration
    else:
      rc = self._nextItems[0]
      del self._nextItems[0]
      return rc
  def next():
    return self.__next__()