.. currentmodule:: pandas
.. ipython:: python :suppress: import os import csv from pandas import DataFrame import pandas as pd pd.options.display.max_rows=15 import numpy as np np.random.seed(123456) randn = np.random.randn randint = np.random.randint np.set_printoptions(precision=4, suppress=True)
For many use cases writing pandas in pure python and numpy is sufficient. In some computationally heavy applications however, it can be possible to achieve sizeable speed-ups by offloading work to cython.
This tutorial assumes you have refactored as much as possible in python, for example trying to remove for loops and making use of numpy vectorization, it's always worth optimising in python first.
This tutorial walks through a "typical" process of cythonizing a slow computation. We use an example from the cython documentation but in the context of pandas. Our final cythonized solution is around 100 times faster than the pure python.
We have a DataFrame to which we want to apply a function row-wise.
.. ipython:: python
df = DataFrame({'a': randn(1000), 'b': randn(1000),'N': randint(100, 1000, (1000)), 'x': 'x'})
df
Here's the function in pure python:
.. ipython:: python
def f(x):
return x * (x - 1)
def integrate_f(a, b, N):
s = 0
dx = (b - a) / N
for i in range(N):
s += f(a + i * dx)
return s * dx
We achieve our result by by using apply (row-wise):
.. ipython:: python %timeit df.apply(lambda x: integrate_f(x['a'], x['b'], x['N']), axis=1)
But clearly this isn't fast enough for us. Let's take a look and see where the time is spent during this operation (limited to the most time consuming four calls) using the prun ipython magic function:
.. ipython:: python %prun -l 4 df.apply(lambda x: integrate_f(x['a'], x['b'], x['N']), axis=1)
By far the majority of time is spend inside either integrate_f or f,
hence we'll concentrate our efforts cythonizing these two functions.
Note
In python 2 replacing the range with its generator counterpart (xrange)
would mean the range line would vanish. In python 3 range is already a generator.
First we're going to need to import the cython magic function to ipython:
.. ipython:: python %load_ext cythonmagic
Now, let's simply copy our functions over to cython as is (the suffix is here to distinguish between function versions):
.. ipython::
In [2]: %%cython
...: def f_plain(x):
...: return x * (x - 1)
...: def integrate_f_plain(a, b, N):
...: s = 0
...: dx = (b - a) / N
...: for i in range(N):
...: s += f_plain(a + i * dx)
...: return s * dx
...:
Note
If you're having trouble pasting the above into your ipython, you may need to be using bleeding edge ipython for paste to play well with cell magics.
.. ipython:: python %timeit df.apply(lambda x: integrate_f_plain(x['a'], x['b'], x['N']), axis=1)
Already this has shaved a third off, not too bad for a simple copy and paste.
We get another huge improvement simply by providing type information:
.. ipython::
In [3]: %%cython
...: cdef double f_typed(double x) except? -2:
...: return x * (x - 1)
...: cpdef double integrate_f_typed(double a, double b, int N):
...: cdef int i
...: cdef double s, dx
...: s = 0
...: dx = (b - a) / N
...: for i in range(N):
...: s += f_typed(a + i * dx)
...: return s * dx
...:
.. ipython:: python %timeit df.apply(lambda x: integrate_f_typed(x['a'], x['b'], x['N']), axis=1)
Now, we're talking! It's now over ten times faster than the original python implementation, and we haven't really modified the code. Let's have another look at what's eating up time:
.. ipython:: python %prun -l 4 df.apply(lambda x: integrate_f_typed(x['a'], x['b'], x['N']), axis=1)
It's calling series... a lot! It's creating a Series from each row, and get-ting from both the index and the series (three times for each row). Function calls are expensive in python, so maybe we could minimise these by cythonizing the apply part.
Note
We are now passing ndarrays into the cython function, fortunately cython plays very nicely with numpy.
.. ipython::
In [4]: %%cython
...: cimport numpy as np
...: import numpy as np
...: cdef double f_typed(double x) except? -2:
...: return x * (x - 1)
...: cpdef double integrate_f_typed(double a, double b, int N):
...: cdef int i
...: cdef double s, dx
...: s = 0
...: dx = (b - a) / N
...: for i in range(N):
...: s += f_typed(a + i * dx)
...: return s * dx
...: cpdef np.ndarray[double] apply_integrate_f(np.ndarray col_a, np.ndarray col_b, np.ndarray col_N):
...: assert (col_a.dtype == np.float and col_b.dtype == np.float and col_N.dtype == np.int)
...: cdef Py_ssize_t i, n = len(col_N)
...: assert (len(col_a) == len(col_b) == n)
...: cdef np.ndarray[double] res = np.empty(n)
...: for i in range(len(col_a)):
...: res[i] = integrate_f_typed(col_a[i], col_b[i], col_N[i])
...: return res
...:
The implementation is simple, it creates an array of zeros and loops over
the rows, applying our integrate_f_typed, and putting this in the zeros array.
Warning
In 0.13.0 since Series has internaly been refactored to no longer sub-class ndarray
but instead subclass NDFrame, you can not pass a Series directly as a ndarray typed parameter
to a cython function. Instead pass the actual ndarray using the .values attribute of the Series.
Prior to 0.13.0
apply_integrate_f(df['a'], df['b'], df['N'])Use .values to get the underlying ndarray
apply_integrate_f(df['a'].values, df['b'].values, df['N'].values)Note
Loops like this would be extremely slow in python, but in Cython looping over numpy arrays is fast.
.. ipython:: python %timeit apply_integrate_f(df['a'].values, df['b'].values, df['N'].values)
We've gotten another big improvement. Let's check again where the time is spent:
.. ipython:: python %prun -l 4 apply_integrate_f(df['a'].values, df['b'].values, df['N'].values)
As one might expect, the majority of the time is now spent in apply_integrate_f,
so if we wanted to make anymore efficiencies we must continue to concentrate our
efforts here.
There is still scope for improvement, here's an example of using some more advanced cython techniques:
.. ipython::
In [5]: %%cython
...: cimport cython
...: cimport numpy as np
...: import numpy as np
...: cdef double f_typed(double x) except? -2:
...: return x * (x - 1)
...: cpdef double integrate_f_typed(double a, double b, int N):
...: cdef int i
...: cdef double s, dx
...: s = 0
...: dx = (b - a) / N
...: for i in range(N):
...: s += f_typed(a + i * dx)
...: return s * dx
...: @cython.boundscheck(False)
...: @cython.wraparound(False)
...: cpdef np.ndarray[double] apply_integrate_f_wrap(np.ndarray[double] col_a, np.ndarray[double] col_b, np.ndarray[Py_ssize_t] col_N):
...: cdef Py_ssize_t i, n = len(col_N)
...: assert len(col_a) == len(col_b) == n
...: cdef np.ndarray[double] res = np.empty(n)
...: for i in range(n):
...: res[i] = integrate_f_typed(col_a[i], col_b[i], col_N[i])
...: return res
...:
.. ipython:: python %timeit apply_integrate_f_wrap(df['a'].values, df['b'].values, df['N'].values)
Even faster, with the caveat that a bug in our cython code (an off-by-one error, for example) might cause a segfault because memory access isn't checked.
- Loading C modules into cython.
Read more in the cython docs.
Expression Evaluation via :func:`~pandas.eval` (Experimental)
.. versionadded:: 0.13
The top-level function :func:`pandas.eval` implements expression evaluation of :class:`~pandas.Series` and :class:`~pandas.DataFrame` objects.
Note
To benefit from using :func:`~pandas.eval` you need to
install numexpr. See the :ref:`recommended dependencies section
<install.recommended_dependencies>` for more details.
The point of using :func:`~pandas.eval` for expression evaluation rather than
plain Python is two-fold: 1) large :class:`~pandas.DataFrame` objects are
evaluated more efficiently and 2) large arithmetic and boolean expressions are
evaluated all at once by the underlying engine (by default numexpr is used
for evaluation).
Note
You should not use :func:`~pandas.eval` for simple expressions or for expressions involving small DataFrames. In fact, :func:`~pandas.eval` is many orders of magnitude slower for smaller expressions/objects than plain ol' Python. A good rule of thumb is to only use :func:`~pandas.eval` when you have a :class:`~pandas.core.frame.DataFrame` with more than 10,000 rows.
:func:`~pandas.eval` supports all arithmetic expressions supported by the engine in addition to some extensions available only in pandas.
Note
The larger the frame and the larger the expression the more speedup you will see from using :func:`~pandas.eval`.
These operations are supported by :func:`pandas.eval`:
- Arithmetic operations except for the left shift (
<<) and right shift (>>) operators, e.g.,df + 2 * pi / s ** 4 % 42 - the_golden_ratio - Comparison operations, including chained comparisons, e.g.,
2 < df < df2 - Boolean operations, e.g.,
df < df2 and df3 < df4 or not df_bool listandtupleliterals, e.g.,[1, 2]or(1, 2)- Attribute access, e.g.,
df.a - Subscript expressions, e.g.,
df[0] - Simple variable evaluation, e.g.,
pd.eval('df')(this is not very useful)
This Python syntax is not allowed:
- Expressions
- Function calls
is/is notoperationsifexpressionslambdaexpressionslist/set/dictcomprehensions- Literal
dictandsetexpressions yieldexpressions- Generator expressions
- Boolean expressions consisting of only scalar values
- Statements
:func:`~pandas.eval` Examples
:func:`pandas.eval` works well with expressions containing large arrays
First let's create a few decent-sized arrays to play with:
.. ipython:: python import pandas as pd from pandas import DataFrame, Series from numpy.random import randn import numpy as np nrows, ncols = 20000, 100 df1, df2, df3, df4 = [DataFrame(randn(nrows, ncols)) for _ in range(4)]
Now let's compare adding them together using plain ol' Python versus :func:`~pandas.eval`:
.. ipython:: python %timeit df1 + df2 + df3 + df4
.. ipython:: python
%timeit pd.eval('df1 + df2 + df3 + df4')
Now let's do the same thing but with comparisons:
.. ipython:: python %timeit (df1 > 0) & (df2 > 0) & (df3 > 0) & (df4 > 0)
.. ipython:: python
%timeit pd.eval('(df1 > 0) & (df2 > 0) & (df3 > 0) & (df4 > 0)')
:func:`~pandas.eval` also works with unaligned pandas objects:
.. ipython:: python s = Series(randn(50)) %timeit df1 + df2 + df3 + df4 + s
.. ipython:: python
%timeit pd.eval('df1 + df2 + df3 + df4 + s')
Note
Operations such as
1 and 2 # would parse to 1 & 2, but should evaluate to 2 3 or 4 # would parse to 3 | 4, but should evaluate to 3 ~1 # this is okay, but slower when using eval
should be performed in Python. An exception will be raised if you try to
perform any boolean/bitwise operations with scalar operands that are not
of type bool or np.bool_. Again, you should perform these kinds of
operations in plain Python.
.. versionadded:: 0.13
In addition to the top level :func:`pandas.eval` function you can also evaluate an expression in the "context" of a :class:`~pandas.DataFrame`.
.. ipython:: python
:suppress:
try:
del a
except NameError:
pass
try:
del b
except NameError:
pass
.. ipython:: python
df = DataFrame(randn(5, 2), columns=['a', 'b'])
df.eval('a + b')
Any expression that is a valid :func:`pandas.eval` expression is also a valid :meth:`DataFrame.eval` expression, with the added benefit that you don't have to prefix the name of the :class:`~pandas.DataFrame` to the column(s) you're interested in evaluating.
In addition, you can perform assignment of columns within an expression. This allows for formulaic evaluation. Only a single assignment is permitted. The assignment target can be a new column name or an existing column name, and it must be a valid Python identifier.
.. ipython:: python
df = DataFrame(dict(a=range(5), b=range(5, 10)))
df.eval('c = a + b')
df.eval('d = a + b + c')
df.eval('a = 1')
df
The equivalent in standard Python would be
.. ipython:: python df = DataFrame(dict(a=range(5), b=range(5, 10))) df['c'] = df.a + df.b df['d'] = df.a + df.b + df.c df['a'] = 1 df
In pandas version 0.14 the local variable API has changed. In pandas 0.13.x, you could refer to local variables the same way you would in standard Python. For example,
df = DataFrame(randn(5, 2), columns=['a', 'b'])
newcol = randn(len(df))
df.eval('b + newcol')
UndefinedVariableError: name 'newcol' is not definedAs you can see from the exception generated, this syntax is no longer allowed.
You must explicitly reference any local variable that you want to use in an
expression by placing the @ character in front of the name. For example,
.. ipython:: python
df = DataFrame(randn(5, 2), columns=list('ab'))
newcol = randn(len(df))
df.eval('b + @newcol')
df.query('b < @newcol')
If you don't prefix the local variable with @, pandas will raise an
exception telling you the variable is undefined.
When using :meth:`DataFrame.eval` and :meth:`DataFrame.query`, this allows you to have a local variable and a :class:`~pandas.DataFrame` column with the same name in an expression.
.. ipython:: python
a = randn()
df.query('@a < a')
df.loc[a < df.a] # same as the previous expression
With :func:`pandas.eval` you cannot use the @ prefix at all, because it
isn't defined in that context. pandas will let you know this if you try to
use @ in a top-level call to :func:`pandas.eval`. For example,
.. ipython:: python
:okexcept:
a, b = 1, 2
pd.eval('@a + b')
In this case, you should simply refer to the variables like you would in standard Python.
.. ipython:: python
pd.eval('a + b')
:func:`pandas.eval` Parsers
There are two different parsers and two different engines you can use as the backend.
The default 'pandas' parser allows a more intuitive syntax for expressing
query-like operations (comparisons, conjunctions and disjunctions). In
particular, the precedence of the & and | operators is made equal to
the precedence of the corresponding boolean operations and and or.
For example, the above conjunction can be written without parentheses.
Alternatively, you can use the 'python' parser to enforce strict Python
semantics.
.. ipython:: python expr = '(df1 > 0) & (df2 > 0) & (df3 > 0) & (df4 > 0)' x = pd.eval(expr, parser='python') expr_no_parens = 'df1 > 0 & df2 > 0 & df3 > 0 & df4 > 0' y = pd.eval(expr_no_parens, parser='pandas') np.all(x == y)
The same expression can be "anded" together with the word :keyword:`and` as well:
.. ipython:: python expr = '(df1 > 0) & (df2 > 0) & (df3 > 0) & (df4 > 0)' x = pd.eval(expr, parser='python') expr_with_ands = 'df1 > 0 and df2 > 0 and df3 > 0 and df4 > 0' y = pd.eval(expr_with_ands, parser='pandas') np.all(x == y)
The and and or operators here have the same precedence that they would
in vanilla Python.
:func:`pandas.eval` Backends
There's also the option to make :func:`~pandas.eval` operate identical to plain ol' Python.
Note
Using the 'python' engine is generally not useful, except for testing
other evaluation engines against it. You will acheive no performance
benefits using :func:`~pandas.eval` with engine='python' and in fact may
incur a performance hit.
You can see this by using :func:`pandas.eval` with the 'python' engine. It
is a bit slower (not by much) than evaluating the same expression in Python
.. ipython:: python %timeit df1 + df2 + df3 + df4
.. ipython:: python
%timeit pd.eval('df1 + df2 + df3 + df4', engine='python')
:func:`pandas.eval` Performance
:func:`~pandas.eval` is intended to speed up certain kinds of operations. In particular, those operations involving complex expressions with large :class:`~pandas.DataFrame`/:class:`~pandas.Series` objects should see a significant performance benefit. Here is a plot showing the running time of :func:`pandas.eval` as function of the size of the frame involved in the computation. The two lines are two different engines.
Note
Operations with smallish objects (around 15k-20k rows) are faster using plain Python:
This plot was created using a DataFrame with 3 columns each containing
floating point values generated using numpy.random.randn().
Expressions that would result in an object dtype or involve datetime operations
(because of NaT) must be evaluated in Python space. The main reason for
this behavior is to maintain backwards compatbility with versions of numpy <
1.7. In those versions of numpy a call to ndarray.astype(str) will
truncate any strings that are more than 60 characters in length. Second, we
can't pass object arrays to numexpr thus string comparisons must be
evaluated in Python space.
The upshot is that this only applies to object-dtype'd expressions. So, if you have an expression--for example
.. ipython:: python
df = DataFrame({'strings': np.repeat(list('cba'), 3),
'nums': np.repeat(range(3), 3)})
df
df.query('strings == "a" and nums == 1')
the numeric part of the comparison (nums == 1) will be evaluated by
numexpr.
In general, :meth:`DataFrame.query`/:func:`pandas.eval` will
evaluate the subexpressions that can be evaluated by numexpr and those
that must be evaluated in Python space transparently to the user. This is done
by inferring the result type of an expression from its arguments and operators.