Python 2.x gotchas and landmines

Question

The purpose of my question is to strengthen my knowledge base with Python and get a better picture of it, which includes knowing its faults and surprises. To keep things sp

Answer 1

Dynamic binding makes typos in your variable names surprisingly hard to find. It's easy to spend half an hour fixing a trivial bug.

EDIT: an example...

for item in some_list:
    ... # lots of code
... # more code
for tiem in some_other_list:
    process(item) # oops!

Answer 2

Loops and lambdas (or any closure, really): variables are bound by name

funcs = []
for x in range(5):
  funcs.append(lambda: x)

[f() for f in funcs]
# output:
# 4 4 4 4 4

A work around is either creating a separate function or passing the args by name:

funcs = []
for x in range(5):
  funcs.append(lambda x=x: x)
[f() for f in funcs]
# output:
# 0 1 2 3 4

Answer 3

Unintentionally mixing oldstyle and newstyle classes can cause seemingly mysterious errors.

Say you have a simple class hierarchy consisting of superclass A and subclass B. When B is instantiated, A's constructor must be called first. The code below correctly does this:

class A(object):
    def __init__(self):
        self.a = 1

class B(A):
    def __init__(self):
        super(B, self).__init__()
        self.b = 1

b = B()

But if you forget to make A a newstyle class and define it like this:

class A:
    def __init__(self):
        self.a = 1

you get this traceback:

Traceback (most recent call last):
  File "AB.py", line 11, in <module>
    b = B()
  File "AB.py", line 7, in __init__
    super(B, self).__init__()
TypeError: super() argument 1 must be type, not classobj

Two other questions relating to this issue are 489269 and 770134

Answer 4

Using class variables when you want instance variables. Most of the time this doesn't cause problems, but if it's a mutable value it causes surprises.

class Foo(object):
    x = {}

But:

>>> f1 = Foo()
>>> f2 = Foo()
>>> f1.x['a'] = 'b'
>>> f2.x
{'a': 'b'}

You almost always want instance variables, which require you to assign inside __init__:

class Foo(object):
    def __init__(self):
        self.x = {}

Answer 5

Floats are not printed at full precision by default (without repr):

x = 1.0 / 3
y = 0.333333333333
print x  #: 0.333333333333
print y  #: 0.333333333333
print x == y  #: False

repr prints too many digits:

print repr(x)  #: 0.33333333333333331
print repr(y)  #: 0.33333333333300003
print x == 0.3333333333333333  #: True