Python Quick Reference
Data types
Strings:
s = "foo bar"
s = 'foo bar'
s = r"c:\dir\new" # raw (== 'c:\\dir\\new')
s = """Hello
world"""
s.join(" baz")
n = len(s)
"Ala ma {} psy i {} koty".format(2,3)
"Square root of 2 is equal to {:.2f}".format(math.sqrt(2))
Lists:
L = [1, 2, 3, 4, 5]
L[0] # single position
L[0:3] # the first three elements
L[-2:] # the last two elements
L[1:4] = [7,8] # substitute
del L[2] # remove elements
L.append(x) # x is a value
L.remove(x)
L.extend(L2) # or: L3 = L + L2
L.pop() # simple stack (with append)
L.sort()
x in L # does L contain x?
L.index(x) # index of the first occurrence
[x*2 for x in L if x>2] # list comprehensions
Tuples:
x = 1,2,3
x = (1,2,3)
x[1]
a,b,c = x
Dictionaries:
D = {'f1': 10, 'f2': 20} # dict creation
D = dict(f1=10, f2=20)
keys = ('a', 'b', 'c')
D = dict.fromkeys(keys) # new dict with empty values
for k in D: print(k) # keys
for v in D.values(): print(v) # values
for k, v in D.items(): # tuples with keys and values
list(D.keys()) # list of keys
sorted(D.keys()) # sorted list of keys
D = {}
D[(1,8,5)] = 100 # 3D sparse matrix
D.get((1,8,5))
D.get((1,1,1), -1)
Sets:
S = {1,3,5}
L = [1, 3, 1, 5, 3]
S = set(L) # set([1, 3, 5])
if (3 in S):
S1+S2, S1-S2, S1^S2, S1|S2
#Loops
for x in range(6): # 0, 1, 2, 3, 4, 5
for x in range(1,6): # 1, 2, 3, 4, 5
for x in range(1,6,2): # 1, 3, 5
for k,v in D.items():
print("D[{}]={}".format(k,v)) # D[f1]=10 D[f2]=20
L = [1, 3, 5]
for i,v in enumerate(L): # (index,value)
for x,y in zip(L1,L2): # returns tuples
for i in sorted(set(L)): print(i) # sorted set from a list
for x in reversed(L1):
#Functions
def foo(arg1, *args, **dic):
"""Example documentation string.
This function does not do anything special.
"""
# arg1 is a positional argument
# args is a list
# dic is a dictionary of named arguments
def foo(a,b,c=0):
L = [1, 2, 3]
foo(*L) # unpacking a list of arguments
D = {'a': 10, 'b': 20}
foo(**D) # unpacking a dictionary of arguments
foo.__doc__ # the docstring
#Input/output
Printing:
str(x) # human readable representation
repr(x) # interpretable representation
File access:
f = open("test.txt", "w") # r / r+ / rb / rb+ / w / wb
f.write("Ala ma kota\n")
f.close()
for line in open("test.txt"): print(line, end="")
L = open("test.txt").readlines() # returns a list of lines
Exclusive access:
f = os.fdopen(os.open("test.txt", os.O_WRONLY|os.O_EXCL), "w")
Input:
x = raw_input("Name: ")
for line in sys.stdin: print(line)
String buffers:
from StringIO import StringIO
buf = StringIO()
sys.stdout = buf
print("Hello")
x = buf.getvalue()
Error stream:
print("Error!", file=sys.stderr, flush=True)
Other file operations:
os.rename(from, to) os.remove(path)
os.chmod(file, 0700) os.stat(file)
Special names
if __name__ == "__main__":
print("Do something")
Exceptions
try:
raise TypeError("arg")
except (RuntimeError, NameError):
pass # empty instruction (NOP)
except:
info = sys.exc_info()
print(info[0])
print(info[1])
traceback.print_tb(info[2])
raise
else:
... # no exception but before finally
finally: # on the way out
... # unhandled exc, release resources
Object-oriented programming
class Person:
ID = 0 # static variable
def __init__(self, name, age=0):
self.name = name
self.age = age
Person.ID += 1
self.ID = Person.ID
def lastName(self):
return self.name.split()[-1]
def __str__(self):
return "{}({},{})".format(self.__class__.__name__,
self.name, self.age)
class Worker(Person):
def __init__(self, name, position, age=0):
super().__init__(name, age)
self.position = position
def __str__(self):
return "{}({},{},{})".format(self.__class__.__name__,
self.name, self.position, self.age)
bob = Worker("Bob Smith", "developer", 25)
print(bob)
Useful APIs
Queues:
Q = collections.deque([10,20,30])
Q.append(40)
Q.popleft()
Pickling:
f = open("myobj.dat", "w")
pickle.dump(x, f)
f = open("myobj.dat", "r")
x = pickle.load(f)
Databases:
conn = sqlite3.connect("data.db")
c = conn.cursor()
c.execute("SELECT * FROM employees")
for row in c:
print(row[0])
conn.commit()
conn.close()
db = shelve.open("file")
db["x"] = y
db.close()
HTTP Server:
srvraddr = ("", 8080) # my hostname, portnumber
srvrobj = BaseHTTPServer.HTTPServer(srvraddr,
CGIHTTPServer.CGIHTTPRequestHandler)
srvrobj.serve_forever()
##URLs:
conn = urllib.urlopen("http://localhost:8080")
reply = conn.read()
Directories:
for (dir, subdirs, files) in os.walk("/tmp"):
for f in files: print(f)
Functional programming
f = lambda x: x+10 # creates an anonymous function
f(5) # returns 15
L = [1, 4, 7]
for x in filter(lambda i: i<5, L): # returns [1, 4]
for x in map(lambda: x: x*2, L): # returns [2, 8, 14]
References
- The Python Tutorial, https://docs.python.org/3/tutorial/
- The Python Language Reference, http://docs.python.org/3/reference/
- The Python Standard Library, http://docs.python.org/3/library/
- PEP-8: Style Guide for Python Code, http://www.python.org/dev/peps/pep-0008/