API: raise/warn SettingWithCopy when chained assignment is detected
```
In [1]: df = DataFrame({"A": [1, 2, 3, 4, 5], "B": [3.125, 4.12, 3.1, 6.2, 7.]})

In [2]: row = df.loc[0]
```

Default is to warn

```
In [3]: row["A"] = 0
pandas/core/generic.py:1001: SettingWithCopyWarning: A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_index,col_indexer] = value instead
  warnings.warn(t,SettingWithCopyWarning)

In [4]: row
Out[4]: 
A    0.000
B    3.125
Name: 0, dtype: float64

In [5]: df
Out[5]: 
   A      B
0  1  3.125
1  2  4.120
2  3  3.100
3  4  6.200
4  5  7.000
```

You can turn it off

```
In [6]: pd.set_option('chained',None)

In [7]: row["A"] = 0

In [8]: row
Out[8]: 
A    0.000
B    3.125
Name: 0, dtype: float64

In [9]: df
Out[9]: 
   A      B
0  1  3.125
1  2  4.120
2  3  3.100
3  4  6.200
4  5  7.000
```

Or set to raise

```
In [10]: row["A"] = 0
SettingWithCopyError: A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_index,col_indexer] = value instead
```

Want!

How would you detect, that a copy was expected? Like first assigning the loc result and then changing it?
 nope just setting a flag on the returned slices is enough when they r copied (2 cases r covered)
 I'm strongly opposed to this - what if you want to do some transformation on a row and then apply it to multiple other rows?  It's far too opinionated and it only covers certain cases, and it doesn't work if you do fancy indexing:

```
In [25]: l = df.loc[[0, 1]]

In [26]: l
Out[26]:
   A  B
0  0  2
1  1  3

In [27]: l["A"] = 5

In [28]: df
Out[28]:
   A  B
0  0  2
1  1  3

In [29]: l
Out[29]:
   A  B
0  5  2
1  5  3
```

It also doesn't work in this case:

```
In [3]: df = pd.DataFrame({"A": [1, 2, 3, 4, 5], "B": [5, 4, 3, 2, 1]}, index=['a', 'b', 'c', 'd', 'e'])

In [4]: df
Out[4]:
   A  B
a  1  5
b  2  4
c  3  3
d  4  2
e  5  1

In [5]: df.loc['a']["A"] = 5
```
 @jtratner you are missing the point here, the last case DOES work. In general if its a single-dtype it WILL work.

This addresses the problem when it DOESN't work and you don't realize that. The problem from a user perspective is that certain operations produce a copy that if you then try to set are actually a copy of the original (this most often happens when you take a cross section that must be converted from a view to a copy)
 I definitely don't understand the intricacies of what makes copies with numpy and pandas.  That said, What I don't like about this is I can't do this:

``` python
df = DataFrame({"A": [1, 2, 3, 4, 5], "B": [3.125, 4.12, 3.1, 6.2, 7.]})
row = df.loc[0]
row["A"] = 0
df - row
```

But I can do it if it's the same dtype. Yes, it's blocking a certain kind of mistake, but how the heck do I make it so that I can do something with row? copy it?

It also means that you _can_ do changed assignment on columns, but can't on rows. (i.e. `df["A"][0]` works).

---

My biggest problem is that it make some Series totally unusable, simply because they were generated by loc.
 Also, in that example I gave, you aren't blocking the mutation of row:

``` python
In [10]: df = DataFrame({"A": [1, 2, 3, 4, 5], "B": [3.125, 4.12, 3.1, 6.2, 7.]})

In [11]: row = df.loc[0]

In [12]: row["A"] = 0
---------------------------------------------------------------------------
SettingWithCopy                           Traceback (most recent call last)
<ipython-input-12-1d82f0d440e7> in <module>()
----> 1 row["A"] = 0

../pandas/core/series.py in __setitem__(self, key, value)
    574     def __setitem__(self, key, value):
    575         try:
--> 576             self._set_with_engine(key, value)
    577             return
    578         except (SettingWithCopy):

../pandas/core/series.py in _set_with_engine(self, key, value)
    627             self.index._engine.set_value(values, key, value)
    628             if getattr(self,'_copy',False):
--> 629                 raise SettingWithCopy("trying to set value on a copy")
    630             return
    631         except KeyError:

SettingWithCopy: trying to set value on a copy

In [13]: df - row
Out[13]:
   A      B
0  1  0.000
1  2  0.995
2  3 -0.025
3  4  3.075
4  5  3.875

In [14]: row
Out[14]:
A    0.000
B    3.125
Name: 0, dtype: float64
```
 This definitely needs to be for 0.14. It's a significant change!
 How about a compromise on this:
1. Make it into a warning
2. Change the warning to mention chained assignment: "Trying to set a value on a copy, which may mean that you aren't setting the value on the object you want. This often occurs because of chained assignment."
3. Offer config option to add a filter for that warning that causes it to raise.
 yeh...maybe a warning is ok....(3 can be done via warnings module)

the issue is that sometimes it work and we allow it, but sometimes it won't. I can in theory
actually make it work (as I can figure out the error), but I think its a bit complicated, because 
I there may be some hard to detect cases, IOW, this its possible this can be a false positive.
 Yeah, I know you can do it via warnings, but if you want to be able to say "If you're starting out with pandas, we recommend you set 'raise_on_setting_copy' to True", which then alters warnings on the callback.
 ok....option is easy
 I'm personally fine with a warning and letting users fiddle, your call on that.  Maybe also performance test?
 no perf hit...this is can an attrib check..

you example above, FYI is technically chained assignment. The bottom line is a cross-section supposed to be a view or not? (e.g. maybe it should NEVER be), even it it COULD be
 @jtratner @JanSchulz ok...new behavior is at top...controlled by the option, default is to warn
 Much happier with this. Do you maybe want to add a note that it won't necessarily catch all chained assignment?
 well...hoping that we do actually catch them and just have some false positives :)

if you have a case that you don't think we catch......
 @wesm ?
 @jtratner ok with putting in?
 Could we change  `_copy` to `_is_copy` and set a default of False on NDFrame? 
 sure
 @jtratner fixed up.....just made a fast access (internal) function
 I thought the version added would be confusing actually because this section was their before
 got it - that makes a lot of sense.
 It would be nice if there would be  API way to remove the warning/raising ("I know what I do") from that object:

```
df = DataFrame({"A": [1, 2, 3, 4, 5], "B": [3.125, 4.12, 3.1, 6.2, 7.]})
row = df.loc[0]
# Here some magic...
row["A"] = 0
# No error/warning
```

It would be even nicer if pandas would be more deterministic, eg. always return a "not view" when the value is assigned but always throw an error when a copy is returned in between two slice operations ("if I assign I want a copy, if I don't assign I want a View"?).
 Just set the config option to None.

the problem is that the underlying decision about whether or not to copy is made by numpy rather than pandas. 
 @JanSchulz being more deterministic is an interesting idea, and could be done in these cases (especially in the case of xs, which I think should _always_ copy), which is the main abuse of the chained setting.

In any event, any more comments; I think this is a nice warning (which is easy to turn off).
 @JanSchulz the basic rule is this

if mixed type then will copy, if a single dtype will provide a view if numpy does (IOW not sure if this is a copy or not, it still _may_ be).
 @jreback What a deterministic rule :-) 

I still find it madness... :-)

``` python
import pandas as pd
df = pd.DataFrame({"A": [1, 2, 3, 4, 5], "B": [3, 4, 3, 6, 7]})
df2 = pd.DataFrame({"A": [1, 2, 3, 4, 5], "B": [3., 4., 3., 6., 7.]})
df.loc[0]["A"] = 0
df2.loc[0]["A"] = 0
print df.ix[0,"A"]
0
print df2.ix[0,"A"]
1
df["A"].loc[0] = 2
df2["A"].loc[0] = 2
print df.ix[0,"A"]
2
print df2.ix[0,"A"]
2
```

This even more so:

``` python
import numpy as np
dfb = pd.DataFrame({'a' : ['one', 'one', 'two','three', 'two', 'one', 'six'],'c' : np.arange(7)})
print dfb.ix[0,"c"]
0
dfb['c'][dfb.a.str.startswith('o')] = 42
print dfb.ix[0,"c"]
42
dfb[dfb.a.str.startswith('o')]['c'] = 43
print dfb.ix[0,"c"]
42
dfb.loc[dfb.a.str.startswith('o'),'c'] = 44
print dfb.ix[0,"c"]
44
```

I see that a view can sometimes be more memory efficient, but this will be (or is) a nightmare for people who come from R, SPSS or excel and have never seen numpy. 

A way to make it more deterministic would be to add a wrapper around all indexer which ensure a view (say for example df.vix) or a copy (df.cix). Then you can at least be explicit what you want to do. Or an option that all default indexer use a "copy or error"-mode and only df.vix and so on a "view or error"-mode. 

So I think the basic rules are:
1.) do not use chained assignments: replace all `df{|.ix|.iloc|.iloc}[column][rows]` by `df.loc[row,col]` (if col and row are both location based) or `df.iloc[row, col]` (if both are integer location based) or `df.ix[row, col]` (if mixed) and you are save.
2.) if you must, first select columns, then rows (to not get a mixed dtype)
3.) if you assign a value to a slice, which you don't want to show up in the original dataframe, make sure that this slice is a copy by ... [don't know how...]
 (3) --> make a copy.

`s = df.ix[a, b].copy()`
 @JanSchulz you rules are sensible....maybe want to expand the chained assignement section even more!

I think this PR will at the very least alert you that you _might_ be doing something you should think about.

Its not hard to eliminate view on cross-section/slicing. I don't think there is any particular benefit to it, unless your data is huge and you are taking a viewable slice (and that's prob why it was done in the first place). 
