scipy butterworth filter has no delay? What's the trick?

Question

On the following graph:

• Orange: original signal
• Blue : butterworth filter from scipy applied.
• Grey : a custom implementation of butterworth filter applied.

On the following graph:

• Orange: same original signal
• Blue: another custom implementation of butterworth filter applied.

Question:

Both custom implementations have delay while scipy's does not have. What's the trick?

Note: Custom implementations don't give the same result because they don't have the same coeff.

Answer 1

I'm using filtfilt function.

Based on Warren Weckesser comment, I went to documentation and it states that

This function applies a linear digital filter twice, once forward and once backwards. The combined filter has zero phase and a filter order twice that of the original.

https://docs.scipy.org/doc/scipy/reference/generated/scipy.signal.filtfilt.html