Backtrader: Candlestick Patterns – Hammer

The article this week comes to us courtesy of a recent suggestion from Rushi. (S)He requested an article for Backtrader that demonstrates how to detect popular candle-stick patterns. Whilst, we won’t be able to cover all candles stick patterns in one article, we can at least make a start! So without further ado, let’s dive in and take a look at the hammer candlestick pattern.

The Hammer

Before we get into the code, let’s spend a bit of time looking at what a hammer or hanging man candle actually is and why some traders believe that these patterns can signal the start of a reversal in price action.

As with most candlestick patterns, the hammer candle derives its name from its appearance. You see, it looks like a hammer!

An image showing the price action of the hammer candle.
A classic hammer candle

This type of candle forms when there was significant selling pressure pushing the price lower. However, that pressure does not last as lots of buyers start to come in and push the price back up to finish much higher than the low of the candle. The result of this is a large lower wick which (not coincidentally) looks like a hammers handle.

Note: In the image above, we show a candle that closed above the open. However, that is not a requirement of a hammer candle. We can still close down, have a significant rebound and still class the candle as a hammer candle.

Why is this a bullish signal?

The general theory is that this pattern signals that selling has been exhausted. There is not enough selling to continue to push the price lower. At the same time, it shows that buyers have started to come back into the picture. Of course, they were always in the picture but the balance has now shifted towards having greater demand than supply.

So in short, this can indicate that:

  1. This level is a level in which buyers gain an appetite and thus, price MIGHT not drop below that level again.
  2. We MIGHT have the start of a significant bounce or trend reversal.

Readers should pay particular attention to the use of might!

Code Scope

As mentioned in the intro. We will focus this article on detecting hammer candles as they happen. We will do this by building a custom indicator that will place a simple mark on the chart when they happen.

The example will focus on the candle pattern itself rather than trying to catch the perfect reversal. In other words, we won’t be checking to see if the price is generally rising or falling before the hammer appears. Similarly, we won’t be making checks to weed out small insignificant hammer candles. I.e The ones with a tiny range but which still meet the overall detection criteria. These are both things that can help improve the quality of your hammer candle and would certainly be worth to consider. However, adding these would ultimately shift us away from the topic in focus. Candle Patterns!

Before we begin

So that the code below can be a complete example, a few parts will be needed to gather data and ingest it into our strategy. The commentary below will not cover these parts. For more information on what is happening read these articles:

  1. For downloading data from Alpha Vantage and ingesting it into Backtrader, read this article.
  2. For creating a strategy with multiple data feeds, read this article.

Of course, the other option is just to extract the indicator portion and insert it into your own strategy.

If you do decide to use the complete example, make sure you find the line the following line and insert your API key.

The script will not work if you don’t.

Example Code

Code Commentary

First, let’s spend a few moments looking at the indicator from a higher level. It has the following lines:

  • Bull_Hammer: This is a line which places markers on the chart. We can configure a line to plot markers using a plot_linesdictionary. The bull hammer line will place a marker anytime we find a hammer candle with a long lower wick.
  • Bear_Hammer: Again, another line which plots markers on the chart. Our bear hammer markers indicate a long upper wick.
  • Signal: This line is not plotted but is available and can be checked in the strategy. This will allow you to monitor it for long and short signals. It will be +1 when a bullish hammer candle is found, -1 when a bearish hammer candle is found and 0 the rest of the time.

Moving on, we get to the magic formula. In fact, the formula used to detect the hammer candle is quite simple in principle (despite the many lines of code!). All we do is take a look at the overall rangeof the candle (highlow). Then we just work out if the wick is greater than a given proportion of the range. The value used is to set the target proportion is rev_wick_ratio and the default value of this parameter is 0.6. This means that the upper or lower wick must be 60% or more of the overall range to be considered a hammer candle.

Why all the lines of code?

The reason we need a few lines to make this calculation is that we need to know how long the wick is. Depending on whether the candle closed up or down means our measurement will change. I.e for the lower wick, if we closed down (red candle) we would measure from the close to the low. Conversely, if it closed up, we would measure from the open to the low. So much of the code within next()is making these checks and then extracting the upper and lower wick ratios accordingly.

Apart from that, we try to catch ZeroDivisionErrorsas this can happen easily if your data is not perfect or you have a candle with no wick.

Finally, we just make the check by dividing the wick ratio by the range of the candle and compare this to our rev_wick_ratio. If we equal or exceed it, we mark update our lines.


When running the strategy you should see an output like this.

showing the output of the example code. This image is zoomed out

Zoom in a little and we can see the candles a bit more clearly

A closer look at the hammer candle detection

As you can see the orange marker indicate a bearish signal and the blue markers indicate a bullish signal.


The idea of using ratios to detect a hammer candle can be applied to many other candlestick types. An engulfing candle is simply a candle with a large body ratio and is larger than the previous candle. With a little thought, you should be able to apply this technique elsewhere with only a few changes.